diff --git a/modules/markup/markdown/markdown.go b/modules/markup/markdown/markdown.go
index 9197dd2fe1..999ae52bb5 100644
--- a/modules/markup/markdown/markdown.go
+++ b/modules/markup/markdown/markdown.go
@@ -7,6 +7,7 @@ package markdown
 
 import (
 	"bytes"
+	"strings"
 	"sync"
 
 	"code.gitea.io/gitea/modules/log"
@@ -57,13 +58,33 @@ func render(body []byte, urlPrefix string, metas map[string]string, wikiMarkdown
 						chromahtml.PreventSurroundingPre(true),
 					),
 					highlighting.WithWrapperRenderer(func(w util.BufWriter, c highlighting.CodeBlockContext, entering bool) {
-						language, _ := c.Language()
-						if language == nil {
-							language = []byte("text")
-						}
 						if entering {
+							language, _ := c.Language()
+							if language == nil {
+								language = []byte("text")
+							}
+
+							languageStr := string(language)
+
+							preClasses := []string{}
+							if languageStr == "mermaid" {
+								preClasses = append(preClasses, "is-loading")
+							}
+
+							if len(preClasses) > 0 {
+								_, err := w.WriteString(`<pre class="` + strings.Join(preClasses, " ") + `">`)
+								if err != nil {
+									return
+								}
+							} else {
+								_, err := w.WriteString(`<pre>`)
+								if err != nil {
+									return
+								}
+							}
+
 							// include language-x class as part of commonmark spec
-							_, err := w.WriteString("<pre><code class=\"chroma language-" + string(language) + "\">")
+							_, err := w.WriteString(`<code class="chroma language-` + string(language) + `">`)
 							if err != nil {
 								return
 							}
diff --git a/modules/markup/sanitizer.go b/modules/markup/sanitizer.go
index e5f6e75084..ba73650bdf 100644
--- a/modules/markup/sanitizer.go
+++ b/modules/markup/sanitizer.go
@@ -38,6 +38,7 @@ func NewSanitizer() {
 func ReplaceSanitizer() {
 	sanitizer.policy = bluemonday.UGCPolicy()
 	// For Chroma markdown plugin
+	sanitizer.policy.AllowAttrs("class").Matching(regexp.MustCompile(`^is-loading$`)).OnElements("pre")
 	sanitizer.policy.AllowAttrs("class").Matching(regexp.MustCompile(`^(chroma )?language-[\w-]+$`)).OnElements("code")
 
 	// Checkboxes
diff --git a/templates/repo/editor/edit.tmpl b/templates/repo/editor/edit.tmpl
index cc98539eab..af65813b33 100644
--- a/templates/repo/editor/edit.tmpl
+++ b/templates/repo/editor/edit.tmpl
@@ -41,9 +41,7 @@
 						data-markdown-file-exts="{{.MarkdownFileExts}}"
 						data-line-wrap-extensions="{{.LineWrapExtensions}}">
 {{.FileContent}}</textarea>
-					<div class="editor-loading">
-						{{.i18n.Tr "loading"}}
-					</div>
+					<div class="editor-loading is-loading"></div>
 				</div>
 				<div class="ui bottom attached tab segment markdown" data-tab="preview">
 					{{.i18n.Tr "loading"}}
diff --git a/web_src/js/markdown/content.js b/web_src/js/markdown/content.js
index f41800ee30..918cd6fe81 100644
--- a/web_src/js/markdown/content.js
+++ b/web_src/js/markdown/content.js
@@ -1,5 +1,5 @@
 import {renderMermaid} from './mermaid.js';
 
 export default async function renderMarkdownContent() {
-  await renderMermaid(document.querySelectorAll('.language-mermaid'));
+  await renderMermaid(document.querySelectorAll('code.language-mermaid'));
 }
diff --git a/web_src/js/markdown/mermaid.js b/web_src/js/markdown/mermaid.js
index 1fda101dc0..a518bc7345 100644
--- a/web_src/js/markdown/mermaid.js
+++ b/web_src/js/markdown/mermaid.js
@@ -1,23 +1,56 @@
-import {random} from '../utils.js';
+const MAX_SOURCE_CHARACTERS = 5000;
+
+function displayError(el, err) {
+  el.closest('pre').classList.remove('is-loading');
+  const errorNode = document.createElement('div');
+  errorNode.setAttribute('class', 'ui message error markdown-block-error mono');
+  errorNode.textContent = err.str || err.message || String(err);
+  el.closest('pre').before(errorNode);
+}
 
 export async function renderMermaid(els) {
   if (!els || !els.length) return;
 
-  const {mermaidAPI} = await import(/* webpackChunkName: "mermaid" */'mermaid');
+  const mermaid = await import(/* webpackChunkName: "mermaid" */'mermaid');
 
-  mermaidAPI.initialize({
-    startOnLoad: false,
+  mermaid.initialize({
+    mermaid: {
+      startOnLoad: false,
+    },
+    flowchart: {
+      useMaxWidth: true,
+      htmlLabels: false,
+    },
     theme: 'neutral',
     securityLevel: 'strict',
   });
 
   for (const el of els) {
-    mermaidAPI.render(`mermaid-${random(12)}`, el.textContent, (svg, bindFunctions) => {
-      const div = document.createElement('div');
-      div.classList.add('mermaid-chart');
-      div.innerHTML = svg;
-      if (typeof bindFunctions === 'function') bindFunctions(div);
-      el.closest('pre').replaceWith(div);
-    });
+    if (el.textContent.length > MAX_SOURCE_CHARACTERS) {
+      displayError(el, new Error(`Mermaid source of ${el.textContent.length} characters exceeds the maximum allowed length of ${MAX_SOURCE_CHARACTERS}.`));
+      continue;
+    }
+
+    let valid;
+    try {
+      valid = mermaid.parse(el.textContent);
+    } catch (err) {
+      displayError(el, err);
+    }
+
+    if (!valid) {
+      el.closest('pre').classList.remove('is-loading');
+      continue;
+    }
+
+    try {
+      mermaid.init(undefined, el, (id) => {
+        const svg = document.getElementById(id);
+        svg.classList.add('mermaid-chart');
+        svg.closest('pre').replaceWith(svg);
+      });
+    } catch (err) {
+      displayError(el, err);
+    }
   }
 }
diff --git a/web_src/less/_markdown.less b/web_src/less/_markdown.less
index 0f57bc4449..1b9c412f6b 100644
--- a/web_src/less/_markdown.less
+++ b/web_src/less/_markdown.less
@@ -495,10 +495,20 @@
     }
 }
 
-.mermaid-chart {
-    display: flex;
-    justify-content: center;
-    align-items: center;
-    padding: 1rem;
-    margin: 1rem 0;
+.markdown-block-error {
+    margin-bottom: 0 !important;
+    border-bottom-left-radius: 0 !important;
+    border-bottom-right-radius: 0 !important;
+    box-shadow: none !important;
+    font-size: 85% !important;
+    white-space: pre !important;
+    padding: .5rem 1rem !important;
+    text-align: left !important;
+}
+
+.markdown-block-error + pre {
+    border-top: none !important;
+    margin-top: 0 !important;
+    border-top-left-radius: 0 !important;
+    border-top-right-radius: 0 !important;
 }
diff --git a/web_src/less/features/animations.less b/web_src/less/features/animations.less
new file mode 100644
index 0000000000..65ff1fef3f
--- /dev/null
+++ b/web_src/less/features/animations.less
@@ -0,0 +1,34 @@
+@keyframes isloadingspin {
+    0% { transform: translate(-50%, -50%) rotate(0deg); }
+    100% { transform: translate(-50%, -50%) rotate(360deg); }
+}
+
+.is-loading {
+    background: transparent !important;
+    color: transparent !important;
+    border: transparent !important;
+    pointer-events: none !important;
+    position: relative !important;
+    overflow: hidden !important;
+}
+
+.is-loading:after {
+    content: "";
+    position: absolute;
+    display: block;
+    width: 4rem;
+    height: 4rem;
+    left: 50%;
+    top: 50%;
+    transform: translate(-50%, -50%);
+    animation: isloadingspin 500ms infinite linear;
+    border-width: 4px;
+    border-style: solid;
+    border-color: #ececec #ececec #666 #666;
+    border-radius: 100%;
+}
+
+.markdown pre.is-loading,
+.editor-loading.is-loading {
+    height: 12rem;
+}
diff --git a/web_src/less/index.less b/web_src/less/index.less
index 33bd41e6f6..ef38f863cd 100644
--- a/web_src/less/index.less
+++ b/web_src/less/index.less
@@ -1,5 +1,7 @@
 @import "~font-awesome/css/font-awesome.css";
 @import "./vendor/gitGraph.css";
+@import "./features/animations.less";
+@import "./markdown/mermaid.less";
 
 @import "_svg";
 @import "_tribute";
diff --git a/web_src/less/markdown/mermaid.less b/web_src/less/markdown/mermaid.less
new file mode 100644
index 0000000000..2b7951eec9
--- /dev/null
+++ b/web_src/less/markdown/mermaid.less
@@ -0,0 +1,12 @@
+.mermaid-chart {
+    display: flex;
+    justify-content: center;
+    align-items: center;
+    padding: 1rem;
+    margin: 1rem 0;
+}
+
+/* mermaid's errorRenderer seems to unavoidably spew stuff into <body>, hide it */
+body > div[id*="mermaid-"] {
+    display: none !important;
+}
diff --git a/web_src/less/themes/theme-arc-green.less b/web_src/less/themes/theme-arc-green.less
index 839cf89b1f..8de66fd251 100644
--- a/web_src/less/themes/theme-arc-green.less
+++ b/web_src/less/themes/theme-arc-green.less
@@ -1260,7 +1260,8 @@ input {
     border-color: #794f31;
 }
 
-.ui.red.message {
+.ui.red.message,
+.ui.error.message {
     background-color: rgba(80, 23, 17, .6);
     color: #f9cbcb;
     box-shadow: 0 0 0 1px rgba(121, 71, 66, .5) inset, 0 0 0 0 transparent;
@@ -1923,3 +1924,12 @@ footer .container .links > * {
 .mermaid-chart {
     filter: invert(84%) hue-rotate(180deg);
 }
+
+.is-loading:after {
+    border-color: #4a4c58 #4a4c58 #d7d7da #d7d7da;
+}
+
+.markdown-block-error {
+    border: 1px solid rgba(121, 71, 66, .5) !important;
+    border-bottom: none !important;
+}