Thousands of people have marched in Thailand's capital and blocked a major road to protest an amnesty bill they say is designed to bring former Prime Minister Thaksin Shinawatra back from overseas exile.

More than 1,000 people blew whistles and temporarily halted traffic in Bangkok's financial district of Silom at lunchtime Monday to protest the legislation, which was approved by the lower house of Parliament on Friday.

Several thousand others joined a peaceful march by the opposition Democrat Party in another part of the city.

The Democrat Party says the legislation is intended to whitewash Thaksin's alleged crimes and allow his return.

After being overthrown in a 2006 military coup, Thaksin fled into exile in 2008 to avoid serving jail time on a corruption conviction.