A coalition of trade unions and community organisations are taking part in the demonstration, which was organised by the People's Assembly.
The group has called for a general election, action on low pay and the repeal of "anti-union" employment laws.
Protesters marched from Embankment to Trafalgar Square, where a rally is being held with various speakers.
The Metropolitan Police said: "There are currently a number of different demonstrations across London this afternoon.
"Several protests groups as expected are present in Trafalgar Square, with stewards on the ground working with police."
The force previously said extra officers would be on the streets as it expected demonstrations by environmental campaign group Just Stop Oil.
Speaking at the rally, former Labour leader Jeremy Corbyn said the government would be "forced to listen" to protesters calling for improved pay and workers rights.
"Governments do eventually listen because of popular movements," he said.
"The people out here are very determined."
Michelle Uden, from south-east London, took her seven-year-old twin boys to the demonstration and said she wanted a change in leadership after struggling with the cost of living.
The 34-year-old cares for her husband, who has epilepsy, she said: "Enough is enough.
"If we don't stand up and fight we'll sit down and cry."
Mrs Uden said she took her children to the protest as she wanted them to see "democracy in action".
Ramona McCartney, national organiser for the People's Assembly, said: "The government is in a deep crisis and the third prime minister in a matter of months has been decided by a tiny elite.
"We want to make this the biggest demonstration possible to force them to a general election and in solidarity with every striking worker."
Laura Pidcock, national secretary of the People's Assembly, said: "We have to come together, as a movement, to organise on the streets and in our communities, and show that our voices will not be silenced and that we want fundamental changes to the way our country is run."