At least six people have been wounded in a bomb blast in the main Burmese city of Rangoon Thursday, the one-year anniversary of the ruling military's brutal crackdown on pro-democracy demonstrators.

Authorities say a small bomb detonated outside Rangoon's City Hall, the focal point of last year's massive protests led by Buddhist monks.  Riot police have sealed off the area from onlookers.

Security has been tightened in Rangoon in advance of this week's anniversary of the crackdown.  The United Nations says at least 31 people were killed and thousands more detained as a result of the crackdown.

In a related story, the rights group U.S. Campaign for Burma marked the anniversary with a statement Thursday saying U.S. activists have organized more than 200 events demanding global action for change in Burma.

The group said the events include meetings, letter-writing campaigns, vigils, and demonstrations in U.S. cities such as New York, Washington, Chicago, and San Francisco.

Some information for this report was provided by AFP and AP.