BANGKOK - Thousands of red shirt supporters of ousted Thai prime minister Thaksin Shinawatra rallied in Bangkok on Saturday in their biggest protest since violent street clashes two months ago.

The United Front for Democracy against Dictatorship (UDD), better known as the red shirts, gathered in the capital to demand that Prime Minister Abhisit Vejjajiva dissolve parliament and call an election.

Six hundred police were on duty and hundreds more on standby at Sanam Luang, a public square near Bangkok's Grand Palace.

Police estimated there were 12,000 protesters, who braved pouring rain in anticipation of a telephone address expected at 9:30 a.m. EDT (1330 GMT) by Thaksin, who was ousted in a 2006 coup and fled Thailand last year to avoid a two-year jail sentence for graft.

This is the kick-off of our campaign to call for the dissolution of the house, one of the UDD's leaders, Jaran Ditthapicha, told the cheering crowd.

The red shirts are supporters of Thaksin and the Puea Thai Party that he backs from exile. He is widely believed to be staying in the United Arab Emirates.

The party's power base is mainly drawn from millions of rural and urban poor who loved Thaksin's populist policies and gave him two landslide election victories.

The UDD staged a peaceful demonstration at Government House for several weeks in April, but the protests intensified when red shirts broke police and military lines and forced the cancellation of an Asian leaders' summit in the town of Pattaya.

A state of emergency was declared a day later and troops were sent to disperse crowds that were blocking a key Bangkok intersection.

The crackdown sparked Thailand's worst street violence in 15 years and severely dented investor confidence.

The UDD says Abhisit is an illegitimate stooge who came to power because of a series of parliamentary defections that they say were engineered by the powerful military.

Abhisit, who was elected in a parliamentary vote in December with the help of former Thaksin allies, has refused to call an election until the battered economy recovers and a process of constitutional reform is complete.

I joined the rally because I want democracy in Thailand, said one protester.

I want Thaksin back because of the way he has been treated. The majority of people can see this injustice.

(Writing by Martin Petty; Editing by Kevin Liffey)

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