Thailand has invoked its Internal Security Act in parts of the capital Bangkok ahead of a series of planned political rallies beginning this weekend.

The security measure will be imposed on three districts of central Bangkok from Thursday until August 10.

It gives security forces the power to block roads, stop vehicles, impose a curfew and ban large gatherings.

A newly-formed coalition of groups called the Thai Peoples' Army will start rallies on Sunday ahead of the introduction of an amnesty bill to parliament which would clear most of those involved in political violence between the 2006 coup and mid 2012.

Opponents claim the government backed bill is designed to allow exiled former prime minister Thaksin Shinawatra, brother of incumbent prime minister Yingluck Shinawatra, to return without facing jail for a corruption conviction.