Many are centred around the Cenotaph war memorial in Whitehall, while others are gathered around the boarded-up statue of Winston Churchill in Parliament Square.
Various groups from around the country, including right-wing activists and groups formed of football supporters, said they had come to London to protect symbols of British history.
Among the demonstrators, who are largely white men, was Paul Golding, leader of the far-right group Britain First, who said they had turned out to "guard our monuments".
The statue of Churchill was boxed up to protect it from potential damage, after protesters daubed "was a racist" on it last weekend.
Protesters sang the national anthem and chanted "England", amid a tense atmosphere and heavy police presence.
One large group moved to barricades outside Downing Street and a number of objects were thrown towards police.