Owen Jones was subjected to homophobic abuse as he spoke at an anti-austerity demonstration.
The Guardian columnist re-tweeted a video which showed several people shouting “homo” towards him during the event in central London.
Jones added that he was branded a “rent boy” by other activists, who appeared to be mounting a counter-demo to the “Britain is Broken” event, partly organised by the People’s Assembly group.
Jones said that in addition to the verbal abuse, those involved attempted to punch and spit at him.
“So, after speaking at today’s [People’s Assembly] rally, I was mobbed by fascists trying to swing punches and who spat down my neck (I know right, yuck, fascist spit!),” he wrote on Twitter.
“Thanks to the comrades who kept them at bay. Again, not intimidated by these [fascists], we are going to defeat you.”
Police officers can be seen in one of the videos standing between Jones and those hurling abuse.
The anti-austerity event was inspired by the “yellow vest” movement in France and saw hundreds of attendees move through the capital.
Crowds wearing yellow vests marched from outside the BBC’s Broadcasting House on Saturday to a rally in Trafalgar Square, where they were addressed by politicians, political activists and trade unionists.
Shadow chancellor John McDonnell told protesters that eight years of austerity is “tearing apart the very social fabric” of the country.
Addressing the issue of Brexit, he said he expects MPs to defeat Theresa May’s deal in Tuesday’s Commons vote.
McDonnell said after that, when “the time is right”, his party will move a motion of no confidence to “bring this government down”.
He said: “It’s now here before us, we could have a socialist prime minister.”
He read out a message from Labour leader Jeremy Corbyn to protesters.
He said: “We need a general election now to bring about the fairer, more equal society we all want to live in and we stand ready to take power.”
Labour MP Laura Pidcock told the crowd that “millions of people are enduring the indignity of poverty”.
She acknowledged there is “division” over Brexit that needs “a solution imminently”.
The MP for North West Durham added: “We are one movement and we are one class against austerity and we must unite on that common fact.”
Protesters chanted anti-government and anti-austerity slogans as the march wound its way through London.
Many people also carried placards and banners with anti-racism and anti-war messages, and some in support of refugees.
One man with a megaphone at the front of the march led chants for a general election, a “fight back” against the government, and for Theresa May to resign.
National organiser Ramona McCartney said the protest was an attempt to “take back the political space taken up by Brexit”.
McCartney said protesters also wanted to show “solidarity with the left and working class in France by wearing the yellow vests today”.
France has seen weeks of nationwide anti-austerity demonstrations led by the “gilets jaune” movement.
Some protesters travelled from France to join the march in London.
Laurie Martin, 26, and Erick Simon, 57, arrived from Normandy on Friday night.
Martin told the Press Association news agency that she came “to support the British because our demands are the same as those fighting austerity in Europe”.
Simon added: “All European countries must join up in this battle against austerity.”
Demonstrator Andrea Hodson, 48, who works in housing support for the homeless, travelled from St Helens to join the protest.
“I’m really angry about the current state of the country, about the way the government is making the cuts,” she said.
“Coming down here and taking action made me feel like I was being more effective.”
She said the yellow vests “originated in France as a people’s movement and this is what this is”.