An explosive device tossed at anti-government protesters wounded 36 in central Bangkok this morning and other violence rippled through the Thai capital after several days of relative calm
At least one protester was seriously hurt.
Police said the device was hurled at protesters marching with their leader, Suthep Thaugsuban, near Chulalongkorn University in the city centre.
"When the incident happened and perpetrators threw the explosive, Suthep was 30 metres away," Akanat Promphan, a spokesman for the movement, said.
Mr Suthep was unharmed.
It was not immediately clear who was responsible and the nature of the device was unknown as forensic experts were prevented from entering the area.
"We have received 12 people who were injured in the blast. One person is seriously injured and the other 11 have sustained various types of injuries, some minor," said an official at Ramathibodi Hospital.
The overall injury toll came from the Erawan Medical Center, which monitors Bangkok hospitals.
The latest unrest flared in November and escalated on Monday when the demonstrators led by Mr Suthep, a former opposition politician, brought parts of the capital to a standstill and forced many ministries to close.
They accuse Prime Minister Yingluck Shinawatra and her brother, former premier Thaksin Shinawatra, of corruption.
They say they want her to step down to make way for an unelected "people's council" to push through broad political reforms.
She has called an election for 2 February and now heads a caretaker administration, but the protesters and opposition parties are boycotting the vote and want her to go immediately.
The protests have been relatively peaceful until now, but sporadic flare-ups between protesters, police and government supporters have left eight people dead and scores injured.
The demonstrations are the biggest since pro-Thaksin protesters paralysed Bangkok in April-May 2010.
That movement ended with a military crackdown and more than 90 people, mostly protesters, were killed during the events.