Two anti-government protesters have been killed and 22 others injured in an attack in the Thai capital, Bangkok, police say.
Witnesses reported explosions and gunfire at a protest camp at the city's Democracy Monument.
Protesters have been pressing the Senate to replace the cabinet with an appointed administration.
It comes after former Prime Minister Yingluck Shinawatra was removed by a Thai court last week.
Thailand has suffered months of deadlock since the protest campaign began in November, with at least 27 people killed and hundreds wounded.
Both pro- and anti-government groups have held rallies in Bangkok, raising fears of further violence.
Reports said grenades were thrown in the latest attack early on Thursday, followed by gunfire. A doctor at an emergency centre in Bangkok said the wounded had been hit by shrapnel.
"The first victim was a protester who was sleeping at Democracy Monument, while the second was a protest guard who died from gunshots," police Major Wallop Prathummuang told AFP news agency.
There have been a number of attacks on the protest movement since it began its street campaign against the government last year, says the BBC's South East Asia correspondent Jonathan Head, but this was on a larger scale than usual.
No group has said it carried out the attack but both pro- and anti-government groups are known to have armed hardliners.
Yingluck - who was removed by the Constitutional Court with several of her ministers - was swiftly replaced by the ruling Puea Thai party. Former Commerce Minister Niwattumrong Boonsongpaisan is now serving as caretaker prime minister.
The party, which won elections in July 2011, says it will push on with a plan to hold another general election in July.
The protesters say they will obstruct this poll in the same way that they blocked an election held in February that was subsequently annulled by the courts.
Thailand has been in the grip of a power struggle since Yingluck's brother, Thaksin Shinawatra, was ousted by the military as prime minister in a 2006 coup.
Thaksin and his family are hated by an urban and middle-class elite who accuse them of corruption and abuse of power.
But Thaksin's policies won him huge support in rural areas, and both the elections since the coup have returned Thaksin-allied governments to power.
The current anti-government protesters want to replace Yingluck's administration with an unelected "people's council" which, they say, would reform the political system.