A man sustained a swollen mouth yesterday when a shouting match between government supporters and opponents turned into a fist fight while caretaker Prime Minister Thaksin Shinawatra was opening an exhibition in Bangkok in honour of His Majesty the King.

"The incident should not have happened in Thailand and shows certain individuals are acting irresponsibly under democratic rule," Thaksin said before his security detail whisked him away.

After arriving at the Siam Paragon shopping centre, Thaksin proceeded to the Lifestyle Hall to inspect the picture books and CDs on royal projects displayed as part of the 60th anniversary celebrations for His Majesty's accession to the throne.

Government supporters and anti-Thaksin protesters started to push and shove and exchange insults as Thaksin walked through the crowd.

A group of about 30 started cursing and chanting for Thaksin's ouster.

"National plunderer, out!" one man shouted, prompting four of Thaksin's supporters in black jackets to start pounding him with their fists.

A melee ensued and the large crowd of onlookers booed both sides for disturbing the peace without any respect for the King.

After Thaksin left, police moved in to persuade the protesters to leave and dispersed the crowd.

Metropolitan Police chief Lt-Colonel Wiroj Chantharangsi said security would be beefed up for Thaksin from now on.

Wiroj presented himself as the top security guard when Thaksin and his wife attended a performance of Khon masked dance at Sala Chalermkrung in the evening.

In Phitsanulok, the People's Alliance for Democracy met with an opposing rally while it organised an anti-Thaksin protest. The PAD chose to rally in the northern province as a countermove after Thaksin completed his inspection tour there.

Pro-government activists led by Boonlert Ruangtim staged a demonstration nearby aimed at disrupting the PAD's activities.

About 1,000 police were deployed to keep peace between the two sides.