The demonstration at the city's Democracy Monument was peaceful, but it was the latest sign of simmering discontent since the army cracked down May 19 to disperse a sprawling anti-government protest camp and end 10 weeks of demonstrations calling for early elections.
Rolling clashes between troops and Red Shirts killed 90 people - mostly protesters shot by soldiers - and injured more than 1400 in the demonstrations' final weeks.
Two grenades have exploded in Bangkok over the past week, killing one bystander and wounding 11. There have been no claims of responsibility but both are presumed linked to ongoing political tensions.
Thai media reported that an unexploded grenade was found in a sewer on the grounds of the prime minister's office compound over the weekend.
Prime Minister Abhisit Vejjajiva's government has kept a state of emergency in place in Bangkok and nine other provinces citing security concerns. The decree bans political gatherings and allows heavy censoring of the media.
The Red Shirt protesters want Abhisit to call early elections. Many of those who camped in Bangkok earlier this year were poor, rural supporters of the deposed fugitive former leader Thaksin Shinawatra, but their calls for more equality in Thailand struck a chord with a wide swath of society who say politicians can no longer ignore the need to address disparities and deep discontent.
Democracy Monument, a traditional site of anti-government demonstrations, was the site of deadly clashes between the so-called Red Shirt protesters and security forces on April 10.
"We are here to let the public know that Red Shirt people died here," said one protester, a 27-year-old artist Anan Wasanarak. "How is the government going to take responsibility for it?"
Police with riot shields watched the protest, which ended peacefully after less than an hour.
