Activists waved the Syrian flag and chanted anti-Israel slogans at a May Day rally addressed by John McDonnell, the Shadow Chancellor.
Thousands of Labour supporters heard Mr McDonnell vow to protect trade union rights if party wins the general election, and read a message from party leader Jeremy Corbyn.
Photographs from the event, in Trafalgar Square on Monday, show the Syrian flag used by Bashar al-Assad’s regime displayed alongside the Communist hammer and sickle symbol, and images of Soviet leader Stalin.
Mr McDonnell’s team were later accused of cropping them out in images he tweeted from the rally.
Twitter user EJ Ward posted a video of activists waving Palestinian and Communist flags, chanting “Labour friends of Israel no more”.
"Labour friends of Israel no more" chant people with flags on Nelson's Column in Trafalgar Square after the May Day March. pic.twitter.com/TrjU7agzsO— EJ Ward (@EJWrites1) May 1, 2017
Labour MP for Bermondsey and Old Southwark Neil Coyle tweeted: “Assad apologists and other Communist airbrushers should have no part in the Labour Party”.
In his speech Mr McDonnell quoted Mr Corbyn, saying: “Workers together won rights to organise together and be represented. The policies we have announced on rights at work this week all have their origins in working class organisations and demands. United, we can win.”
He ended his speech by adding: “This is our opportunity, brothers and sisters. Some of us have worked for all of our lives. The message is this: carpe diem – seize the moment. This is our chance. Take it, brothers and sisters. Solidarity.”
The Labour Party has been approached for comment.