Celebrities including Charlotte Church and Russell Brand are joining up to 70,000 people in what is predicted to be the biggest demonstration for years.
The People's Assembly, which is organising the demonstrations, say they are a protest against austerity, spending cuts and nuclear weapons.
Thousands are heading towards Westminster where one of the rallies will be held. Politicians and celebrities who will address the crowds include Charlotte Church, comedian Russell Brand, Len McCluskey, general secretary of Unite, Labour leadership contender Jeremy Corbyn, and Sinn Fein's Martin McGuinness.
Others taking part include actors Michael Sheen, Maxine Peake and former Coronation Street star Julie Hesmondhalgh.
Hollywood actor Sheen, who has played Tony Blair, said: "We must not let 'austerity measures' provide a cover for a systematic dismantling of the mechanisms of equality in this country.
"The truth is that the financial crisis was caused by the unchecked desires for profit at the expense of responsibility in the financial sector.
"The truth is that the danger of 'welfare handouts' is a myth, whereas the massive banking bailouts were a grotesque reality.
"Together we must use our democratic right to make our voices heard and speak truth to power."
Charlotte Church said: "I'll be there in London on 20th June, not just because it'll be loads of fun, but because staying at home and doom-mongering doesn't do anyone any good. The government are using diversionary tactics to distract us from the burglary that's about to take place. Fox hunting is not something we need to be talking about. The Transatlantic Trade and Investment Partnership is!
"We need to show the government that we see through the lies they've told us about the state of our economy. Austerity is but another shiny bauble to bewilder us with, and yet the cuts that are taking place threaten, not just livelihoods, but lives. This government is extreme in it's ideology and is a genuine threat to our freedoms, our privileges and our economic future. We have a democratic responsibility to oppose this government, and we can sing songs while we do it. Let's do something good!"
Sam Fairbairn of the People's Assembly, told Sky News: "It will be the start of a campaign of protest, strikes, direct action and civil disobedience up and down the country.
"We will not rest until austerity is history, our services are back in public hands and the needs of the majority are put first."
Kate Hudson, general secretary of the Campaign for Nuclear Disarmament, said mass mobilisation was key to persuading the new Government not to renew the Trident nuclear weapons.
Lindsey German, convenor of the Stop The War Coalition, said: "I will be marching because I am fed up with all the main parties saying they are going to promote austerity. "They have made the rich richer and the poor poorer and they're going to continue."
The Conservative government are preparing for another £25bn of spending cuts, including reducing £12bn from the social security budget.