Witnesses reported explosions and gunfire early on Thursday at a protest camp at Bangkok’s Democracy Monument.
Protesters have been pressing the Senate to replace the cabinet with an appointed administration.
Later on Thursday, they forced a meeting between the government and the Election Commission to be abandoned.
The government is trying to organize a new general election in July, after protesters disrupted the previous election in parts of the country.
A crowd led by Suthep Thaugsuban, head of the anti-government movement, broke into the Air Force base where the meeting between acting PM Niwatthamrong Boonsongphaisan and the commission was being held.
“The meeting is over, the prime minister is leaving. We cannot continue today,” a member of the commission was quoted by Reuters news agency as saying.
The Election Commission on Thursday has called for polls to be postponed due to the political unrest, AFP news agency says, citing officials.
The attack on protesters comes days after former PM Yingluck Shinawatra was removed by a Thai court.
Reports said grenades were thrown in the latest attack in the early hours of Thursday, followed by gunfire. A doctor at an emergency centre in Bangkok said the wounded had been hit by shrapnel.
Police identified two of the victims as a protester who was asleep and a protest guard who was shot.
There have been a number of attacks on the protest movement since it began its street campaign against the government last year.
No group has said it carried out the attack but both pro- and anti-government groups are known to have armed hardliners.
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