The Red Shirts, who believe Prime Minister Abhisit Vejjajiva came to power undemocratically, regularly hold mass rallies in shows of strength after their militant two-month street sit-in was smashed last May by the military. A total of 91 people were killed and more than 1,400 hurt in a series of escalating confrontations after Abhisit rebuffed their demand that he step down and call immediate elections.

Last April 10, 20 civilians, including a Japanese cameraman for the Reuters news agency, and five members of the security forces, including an army colonel, died when soldiers attempted to clear a Red Shirt rally site. The soldiers became bogged down as night fell and were attacked by protesters supported by gunmen dressed in black whose weapons included M79 grenade launchers.

Sunday’s demonstration, at the same place as last year’s clash, memorialized the dead protesters, but also amounted to a pre-campaign rally ahead of general elections that Abhisit has pledged to hold by early July.

The crowd, estimated initially by police at about 10,000, roughly doubled as evening fell.

Many Red Shirt protesters are poor, rural supporters of former Prime Minister Thaksin Shinawatra, who was overthrown in a 2006 military coup. The protesters complain of a lack of justice in Thailand and double standards for the ruling elite and the rest of the country. Thaksin, who was convicted of corruption after the coup and is in exile, was to address the crowd Sunday night by a video link.

Critics have accused the government of stalling the investigation of protesters’ deaths, saying it fears a backlash from the public if findings show that soldiers killed civilians.

"The 91 deaths should not have occurred in Thailand. During the course of fighting for democracy, nobody should die, either protesters or soldiers," said Jatuporn Prompan, a Red Shirt leader. "On this one-year anniversary, the military should know what they are doing. They should stop intervening in the democratic process."

Among those attending Sunday’s rally was Sangwan Tipprom, 38, an employee of a florist shop in Bangkok. Her husband Praison was fatally shot last April 10, leaving her to raise their 8-year-old son and 4-year-old daughter.

"I still want to know who killed my husband and I want to ask Abhisit Vejjajiva why he had to be killed," she said.

The main government inquiry into the violence, by the Department of Special Investigation — the Thai equivalent of the U.S. FBI — has failed to establish blame for most of the deaths.