It follows the release on bail of seven top leaders in February, after nine months in jail on terrorism charges.

Those leaders addressed a large rally on Saturday to mark one year since the start of their Bangkok protests which ended violently on May 19 last year.

The latest releases come after Thai Prime Minister Abhisit Vejjajiva's said he intended to call early elections.

The four protest leaders freed on bail had been held on terrorism and other charges relating to last year's protest movement led by the United Front for Democracy against Dictatorship (UDD).

The two-month protest ended last May after a violent confrontation with government forces, which left 91 people dead and more than 1,400 injured.

Several hundred protesters were also arrested.

Several other red-shirt leaders, including the popular singer Arisman Pongruangrong, have not yet surrendered.