The Thai authorities tightened security at the Constitutional Court on Thursday as judges set to start two-day witness testimony on constitutionality of charter amendment.

The court will deliberate whether the government-sponsored amendment bill deemed unconstitutional and whether the amendment bill is an attempt to topple democratic rule with the King as head of state or to grab power by unconstitutional means, as alleged by the petitioners.

The petitioners to the Constitutional Court comprises a group of Democrat MPs and pro-establishment activists.

The amendment bill, sponsored by ruling Pheu Thai Party, is widely seen as seeking to whitewash fugitive former prime minister Thaksin Shinawatra who was ousted in a coup in 2006 and regarded as the de facto leader of the ruling party.

On Wednesday, the court approved 15 witnesses at hearings on Thursday and Friday.

The court began hearing witness testimony at 9.30 a.m. local time Thursday. Seven witnesses opposed to the bill will testify Thursday, while eight witnesses supporting the measures will testify on Friday.

The court also selected two legal experts from each side.

According to Thai News Agency, the justices are unlikely to make any decision on Friday.

On Wednesday morning, about 200 pro-establishment activists convened in front of the Constitutional Court to show moral support for the judiciary.

Later in the afternoon, about 20 pro-Thaksin Red-shirt activists led by Sombat Boonngamanong held a brief protest against the court. Sombat showed the court a "red card" as used by the referee in a football match.