Thousands of people are marching through central London as part of an anti-austerity demonstration, the BBC News reported.
The march began after a rally outside the Bank of England and will end with another rally outside the Houses of Parliament - with organizers saying around 75,000 people could attend.
The government said austerity measures are vital to cut the deficit.
But Labor leadership contender Jeremy Corbyn said that austerity was a "cover" for deepening inequality.
The MP told BBC Radio 4's Today programme his party had been wrong to accept the Conservatives' "cuts agenda" during the general election campaign.
Corbyn, one of four candidates to succeed Ed Miliband, said Labor instead needed to "rebalance the social direction" of the country.
Organizers said other speakers will include singer Charlotte Church, Northern Ireland's Deputy First Minister Martin McGuinness, former Coronation Street actress Julie Hesmondhalgh, and comedian and activist Russell Brand.
Thousand of people are also attending a separate rally in Glasgow, organized by Scotland United Against Austerity.