BANGKOK - A BLAST wounded 13 people in Thailand's capital on early Thursday when assailants hurled an explosive device at market vendors who had gathered to protest a rent hike at the government-owned facility, police said.

The blast occurred in the Thai capital at around 1am (12am, Singapore time) when some protesters were sleeping in makeshift tents while others gathered outdoors, police Colonel Sutip Palitkusontap said.

Two of the protesters remained hospitalised with serious injuries, but the rest have been released, said Mr Surachet Sathitniramai at the Narenthorn Medical Centre.

Hundreds of vendors who operate stalls at the outdoor market in Bangkok have been staging a protest since Wednesday against Legal Professional, the facility's new privately contracted management company, Mr Sutip said.

Two men were seen dropping a plastic bag from a fly-over bridge onto the protest side at an intersection in Klong Toey district before the blast occurred, Mr Sutip said, citing witnesses.

'It remains unclear what kind of explosive device it was and who was behind the attack,' he said.

There was no claim of responsibility for the attack and no indication that the blast was related to Thailand's ongoing political strife.

Protesters have been camping out for months at the Thai prime minister's office in hopes of persuading the government to resign.

The market's land is owned by the Port Authority of Thailand, which recently hired new management for the market.

The protesters allege the bidding process for the hiring was not transparent, and that they have been treated unfairly by the new management, which has sharply raised rents without improving conditions.

Officials at the Port Authority of Thailand and the company could not immediately be reached for comment. -- AP