"Thaksin fled the indictment to London and is planning to seek asylum in Britain," the letter said.

"If Thaksin is not brought back to face the charges against him in Thai courts, it will further contribute to the political and democratic deadlock."

A spokesman from the British embassy told AFP that the letter would be passed on to London.

"Extradition requests are decided in strict accordance with the law," he said.

Colonel Rapeepong Supornsri, deputy commander of Bangkok's Metropolitan Police, said up to 8,000 protesters flooded the streets, with no violence reported. The protesters began to disperse at lunchtime.

The demonstrators from the so-called People's Alliance for Democracy (PAD) wore yellow T-shirts and headscarfs -- a show of allegiance to the king -- and carried signs in English and Thai.

"Thaksin is not a political refugee but a criminal evading an arrest warrant," one banner hoisted above the crowd read.

Thaksin and Pojaman skipped a required court appearance in a corruption case last Monday and instead fled to Britain, with Thaksin saying he believed political interference in Thailand would rob him of a fair trial on graft charges.

A warrant has been issued for his arrest, and the Thai attorney general's office has begun considering a possible bid to extradite Thaksin from Britain, where he owns the Premier League football club Manchester City.

Thaksin was overthrown in a coup in September 2006 and returned to Thailand in February this year after 18 months of self-imposed exile, vowing to fight the corruption charges.

But instead the cases mounted, and on July 31 his wife was convicted of tax evasion and sentenced to three years in jail.

The PAD has been holding street protests since May demanding the resignation of premier Samak Sundaravej, who comfortably won elections in December last year. They accuse Samak of being a puppet of his old ally Thaksin.