Police estimated around 25,000 protesters massed in the city as part of a rally that crossed from Bangkok's retail heart to Democracy Monument, both sites of bloodshed during last year's April and May demonstration.

Dressed in their trademark colour and waving banners, flags and plastic clappers, the crowd cheered loudly as the movement's key figures spoke to protesters.

"We came today to ask for justice for our people who died exactly nine months ago at this place," said Thida Thavornseth, leader of Red Shirt movement.

More than 90 died and nearly 1,900 were injured in clashes between protesters and the army during the two-month rallies in 2010 calling for immediate elections.

Thailand's political landscape has remained fractured since the unrest.

The mainly rural, working class Reds are broadly loyal to fugitive former premier Thaksin Shinawatra, who was ousted in a military coup in 2006 and lives overseas to avoid a jail sentence for corruption imposed in absentia.

They view Thailand's current government as undemocratic because it came to power in 2008 in a parliamentary vote with the backing of the army after a court ruling threw out the previous administration.

Many key Red Shirt leaders remain in prison on terrorism charges and the protest movement has held a series of peaceful one-day rallies in the capital in recent weeks demanding their release.