Witnesses say an explosion went off in Myanmar’s main city amid tight security to mark the one-year anniversary of the junta’s crackdown on pro-democracy protesters.

Witnesses say three people appeared to have been lightly wounded by Thursday’s blast.

The explosion occurred around 10:30 a.m. local time at a bus stop near Yangon’s City Hall, which was central to the protests led by Buddhist monks a year ago.

Riot police have poured into the area and sealed it off with yellow tape. Security has been tight in downtown Yangon ahead of the anniversary.

At least 31 people were killed when troops fired on protesters Sept. 26-27, 2007, violently crushing the peaceful uprising against the military government.