BANGKOK (AFP) - Thailand's senate unanimously rejected a contentious political amnesty bill late on Monday, the deputy speaker said, as tens of thousands of anti-government protesters massed on Bangkok's streets.

"This house rejects this bill for consideration," he said, adding that all of the 141 senators present voted against the legislation, which critics say was aimed at helping divisive former premier Thaksin Shinawatra return to Thailand from self-exile.

The decision kicks the bill, which has provoked nearly a fortnight of protest on the capital's streets, back to the lower house, where under pressure Prime Minister Yingluck Shinawatra - Mr Thaksin's sister - has already vowed to kill it.

Government opponents say they do not believe Ms Yingluck will stick to her word, but either way the bill - or similar bills - cannot be sent back to the senate for consideration for 180 days.

"If the senate rejects any bill... (it) will be suspended and the draft will be sent back to house of representatives which cannot bring this bill back for re-consideration before 180 days," the deputy speaker added.

Critics say the legislation was crafted to pave the way for the return of the polarising ex-premier Thaksin, who is a hate figure for many among the Bangkok middle and upper classes.

An estimated 50,000 protesters remained in the city's political centre on Monday night, according to police, heightening fears of clashes with authorities as street politics appeared to return to the capital, which has been rocked by bouts of unrest since Mr Thaksin was ousted in an army coup in 2006.