diff --git a/telethon/client/auth.py b/telethon/client/auth.py
index b4a1c74a..9fcb1bce 100644
--- a/telethon/client/auth.py
+++ b/telethon/client/auth.py
@@ -483,6 +483,12 @@ class AuthMethods(MessageParseMethods, UserMethods):
     # region with blocks
 
     def __enter__(self):
+        if self._loop.is_running():
+            raise RuntimeError(
+                'You must use "async with" if the event loop '
+                'is running (i.e. you are inside an "async def")'
+            )
+
         return self.start()
 
     async def __aenter__(self):
diff --git a/telethon/tl/custom/conversation.py b/telethon/tl/custom/conversation.py
index 97b6816a..04f3ad5e 100644
--- a/telethon/tl/custom/conversation.py
+++ b/telethon/tl/custom/conversation.py
@@ -379,6 +379,12 @@ class Conversation(ChatGetter):
                 fut.cancel()
 
     def __enter__(self):
+        if self._client.loop.is_running():
+            raise RuntimeError(
+                'You must use "async with" if the event loop '
+                'is running (i.e. you are inside an "async def")'
+            )
+
         return self._client.loop.run_until_complete(self.__aenter__())
 
     async def __aenter__(self):