Tens of thousands of people have protested in London, Glasgow and Belfast about pay and austerity, BBC said in a report.
Many of those protesting were public sector workers such as teachers and nurses opposed to a below-inflation 1% pay offer from the government.
The "massive turnout" will send a strong message to Downing Street, the Trades Union Congress (TUC) general secretary Frances O'Grady has said. The government says pay restraint has safeguarded jobs and services.
Public sector workers including teachers, nurses, civil servants and hospital workers are among those taking part in the protests, alongside rail and postal workers and others from private firms. The marches follow public sector strikes earlier this week.
The TUC, which organized the protests under the slogan "Britain Needs a Pay Rise", said between 80,000 and 90,000 people were taking part in the London march.
Dave Prentis, general secretary of the Unison union, said the "best thing" the government could do was "recognize the value of the masses of people here today who have suffered and give them a pay rise".
"Our members didn't cause this recession, our members didn't cause the failures of the banks," he said.
The TUC says average wages have fallen by £50 a week in real terms since 2008.
The union's general secretary Frances O'Grady said, "Our message is that after the longest and deepest pay squeeze in recorded history, it's time to end the lock-out that has kept the vast majority from sharing in the economic recovery."
Len McCluskey, general secretary of the Unite union, said Labor party should support workers by offering a "clear socialist alternative" to the Conservatives at the next election.
"I say to Labor - stop being scared of your own shadow. Don't shrink what you offer the British people," he said.
