Crowds have now moved on to Trafalgar Square where some demonstrators have been sat in the road blocking traffic for up to an hour.
Scotland Yards says no arrests have been made.
Labour shadow chancellor John McDonnell labelled Boris Johnson a “dictator” in a stinging attack as he addressed crowds outside Downing Street.
Referencing Jeremy Corbyn’s absence from the rally — which earlier prompted cries of “where’s Jeremy?” from protesters — McDonnell said the brought a “message of solidarity” from the Labour leader.
He went on to attack the prime minister’s move to suspend Parliament, saying it had had “rightfully” been called “a very British coup”, adding: “We have defeated dictators in the past in our history and we will defeat this dictator under Johnson.”
Earlier, he said: “This is a fight to protect our democracy. We know what Johnson is up to, it’s not very subtle, is it? He wants to close down our democracy to force through a no-deal Brexit.”
He added: “He also wants to try and convene a general election based upon this idea of his, he can frame it as people versus Parliament. Let’s make it absolutely clear… Boris Johnson, this is not about Parliament versus the people, this is about you versus the people.”
Meanwhile, Shadow women and equalities minister Dawn Butler, wearing a “defend democracy” sticker, geed up the crowd, chanting: “Wanna know what democracy looks like? This is what democracy looks like.”