A blast wounded 13 people in Thailand's capital 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 1 a.m. local time when some protesters were sleeping in makeshift tents while others gathered outdoors, Pol. Col. Sutip Palitkusontap said. Twelve of the injured were hospitalized, two of them with serious injuries, he said.

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

Two men were seen dropping a plastic bag from a fly-over bridge to the protest side at an intersection in Klong Toey district before the blast occurred, 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 continuing political strife, which has included protesters camping out outside the prime minister's offices.

The market's land is owned by 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.

Officials at the Port Authority of Thailand could not immediately be reached for comment.