A motorcyclist threw a home-made grenade into an anti-government "yellow shirt" rally in central Bangkok late on Tuesday, injuring two men, police said.
The unidentified assailant, wearing a crash helmet, sped away after the attack on supporters of the People's Alliance for Democracy (PAD) who have been camped out near Prime Minister Abhisit Vejjajiva's office for more than four months, protesting against his handling of a border dispute with Cambodia.
A PAD supporter and an ice-cream seller were wounded by shrapnel near a makeshift stage used by PAD leaders to address supporters.
The PAD is a nationalist, royalist movement that in the past spearheaded mass protests against former Prime Minister Thaksin Shinawatra, prior to his removal in a bloodless military coup?in 2006.
It also led street protests against subsequent pro-Thaksin governments and occupied Bangkok's two main airports for a week in late 2008, ending the siege when the courts dissolved the ruling party, paving the way for Abhisit to form a coalition.
Thailand holds a general election on July 3. After falling out with Abhisit, the PAD set up its own party, which will field candidates in the election, but the organisation is in some disarray. The leaders of the broader "yellow shirt" movement want their supporters to vote "no" on their ballot paper, effectively rejecting all the candidates.
The rival "red shirts", whose protest in Bangkok from March to May last year was put down by the military, support the pro-Thaksin Puea Thai Party.
Campaigning for the election has been largely peaceful so far and the police lifted a tough security law on May 24, taking the view that political violence was unlikely and normal legislation would be sufficient to ensure order.