The Government has been criticised for "destroying" communities with its spending cuts and other policies as thousands of trade unionists, pensioners, students and activists took part in the annual May Day celebrations.
Leaders of unions representing health workers, council staff and civil servants told a rally in London's Trafalgar Square that the coalition's policies had failed, leading to rising unemployment and cuts in public services.
Len McCluskey, general secretary of Unite, said: "We are not prepared to stand by and watch ordinary working people pay the price of a crisis caused by spivs and speculators and greedy bankers.
"Of course there is an alternative to the cuts. We should be investing in communities, not destroying them."
Mr McCluskey paid tribute to the Occupy and UK Uncut direct action groups and said that every form of civil disobedience and direct action should be supported.
Mark Serwotka, leader of the Public and Commercial Services union, predicted that up to half a million civil servants, lecturers, health workers, Ministry of Defence staff and Royal Fleet Auxiliary employees will take industrial action on May 10 in the bitter dispute over the Government's controversial public sector reforms.
"Some workers will be going on strike for the fourth or fifth time because they know that if they lose their pensions now, they will never get them back."
Members of the Occupy group handed out thousands of flowers to passengers at London's Liverpool Street station, bearing the message: "There Is Something Better Out There" as part of a day of activities across the capital.
Rallies and marches were held in many parts of the country, supported by a wide variety of trade unions and other groups, as well as disabled workers from the Remploy factories facing closure.