The biggest demonstration was held in London, where a rally outside the Bank of England marched towards the Houses of Parliament.
Union leaders and British celebrities including singer Charlotte Church and actor Russell Brand addressed crowds.
Protests also took place in the cities of Liverpool and Glasgow against the David Cameron-led government's austerity measures which it claims are vital to cut the country's deficit.
The London protest was organised by protest group the People's Assembly, while the rally in the Scottish capital of Glasgow was organised by Scotland United Against Austerity.
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Some of the placards read: 'A living wage for Mothers' and 'We're mad as hell and we're not taking it any more'.
Opposition Labour leadership contender Jeremy Corbyn said that austerity was a "cover" for deepening inequality.
"Britain has become a more unequal society, is becoming a more unequal society and austerity is a cover for actually deepening that level of inequality," he told BBC.
But a UK's Treasury department spokesperson said income inequality in the UK was lower than in 2010.
As the protest marches took place, Cameron, in a Facebook post, vowed "not to waste a second in delivering our manifesto commitments".
"We will keep working through our plan to create more security and opportunity in our country -- and, with your help, we can secure a brighter future for everyone in Britain," he said.
Cameron triumphed in an unexpected election result on May 7 that gave his centre-right Conservative party a majority in parliament for the first time in nearly 20 years. He had previously led a coalition government since 2010.