Thailand has been bitterly divided since Mr Thaksin was ousted in a military coup in 2006, and the proposal re-ignited simmering political tensions.
Groups of protesters camped out at the foreign and finance ministries overnight. On Tuesday, they surrounded the interior, tourism, transport and agriculture ministries.
About 1,000 protesters gathered outside both the finance and interior ministries, blowing whistles and chanting "get out!", reports said.
Akanat Promphan, a protest spokesman, said: "We are occupying the finance ministry in a non-violent and peaceful way, so our supporters around the country can do the same and occupy all government offices.