The legislation is expected to sail through its first reading.
Thaksin was ousted by the military in 2006 and found guilty of abuse of power. He fled the country before a two-year sentence was pronounced for him.
Observers believe that his sister Yingluck Shinawatra, who is now the prime minister, wants to find a way to quash his conviction so that he can return from exile without having to go to jail.
Later, the march was dispersed by the police and Democrat leader asked Abhisit Vejjajiva the protesters to return home.
Vejjajiva told party TV channel Blue Sky News that the protest was not about organising a mob but about spreading the truth to the people, to recognise the dangers of this amnesty Bill which had different hidden agendas.
Thaksin, who is popular among the rural people, is not trusted by the military-royalist establishment who feel that he is corrupt and authoritarian.