diff --git a/core/modules/block/block.install b/core/modules/block/block.install
index ee72e0f..56b91c3 100644
--- a/core/modules/block/block.install
+++ b/core/modules/block/block.install
@@ -324,6 +324,36 @@ function block_update_8003() {
 }
 
 /**
+ * Rename default menu names.
+ */
+function block_update_8004() {
+  // System menu's new block deltas are prefixed with 'menu-'.
+  $map = array(
+    'navigation' => 'menu-tools',
+    'management' => 'menu-admin',
+    'user-menu' => 'menu-account',
+    'main-menu' => 'menu-main',
+  );
+  foreach ($map as $old => $new) {
+    db_update('block')
+      ->condition('module', 'system')
+      ->condition('delta', $old)
+      ->fields(array('delta' => $new))
+      ->execute();
+    db_update('block_language')
+      ->condition('module', 'system')
+      ->condition('delta', $old)
+      ->fields(array('delta' => $new))
+      ->execute();
+    db_update('block_role')
+      ->condition('module', 'system')
+      ->condition('delta', $old)
+      ->fields(array('delta' => $new))
+      ->execute();
+  }
+}
+
+/**
  * @} End of "addtogroup updates-7.x-to-8.x".
  * The next series of updates should start at 9000.
  */
diff --git a/core/modules/node/node.install b/core/modules/node/node.install
index a9c2c36..570cb26 100644
--- a/core/modules/node/node.install
+++ b/core/modules/node/node.install
@@ -692,6 +692,26 @@ function node_update_8007() {
 }
 
 /**
+ * Rename default menu names.
+ */
+function node_update_8008() {
+  // System menu's new block deltas are prefixed with 'menu-'.
+  $map = array(
+    'navigation' => 'menu-tools',
+    'management' => 'menu-admin',
+    'user-menu' => 'menu-account',
+    'main-menu' => 'menu-main',
+  );
+  foreach ($map as $old => $new) {
+    db_update('block_node_type')
+      ->condition('module', 'system')
+      ->condition('delta', $old)
+      ->fields(array('delta' => $new))
+      ->execute();
+  }
+}
+
+/**
  * @} End of "addtogroup updates-7.x-to-8.x"
  * The next series of updates should start at 9000.
  */
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index a9d951b..7f9f33a 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -2515,11 +2515,13 @@ function system_block_info() {
     'status' => 1,
   );
   // System-defined menu blocks.
+  // The block deltas need to be prefixed with 'menu-', since the 'main' menu
+  // would otherwise clash with the 'main' page content block defined above.
   foreach (menu_list_system_menus() as $menu_name => $title) {
-    $blocks[$menu_name]['info'] = t($title);
+    $blocks['menu-' . $menu_name]['info'] = t($title);
     // Menu blocks can't be cached because each menu item can have
     // a custom access callback. menu.inc manages its own caching.
-    $blocks[$menu_name]['cache'] = DRUPAL_NO_CACHE;
+    $blocks['menu-' . $menu_name]['cache'] = DRUPAL_NO_CACHE;
   }
   return $blocks;
 }
@@ -2546,6 +2548,8 @@ function system_block_view($delta = '') {
       $block['content'] = menu_get_active_help();
       return $block;
     default:
+      // Strip off 'menu-' prefix from block delta.
+      $delta = substr($delta, 5);
       // All system menu blocks.
       $system_menus = menu_list_system_menus();
       if (isset($system_menus[$delta])) {