Police in riot gear arrested at least 50 protesters as the U.S. Democratic National Convention opened in the western U.S. city of Denver, Colorado.

Police fired tear gas and clashed with demonstrators late Monday, as authorities tried to disperse some 300 protesters gathered in front of a local government building, the Denver City and County Building. 

Authorities say the protesters were disrupting traffic and blocking public roads. 

The clashes took place about a mile from the site of the Democratic National Convention. Hundreds of activists have converged in the area to protest issues like the war in Iraq and climate change.

Protesters held the first of several planned rallies Sunday, with a peaceful march through downtown Denver.  Hundreds of demonstrators took to the heavily-patrolled streets, waving signs and chanting, "Stop the torture, stop the war."

Authorities have deployed riot police and secret service officers to secure the perimeters of the convention sites, where tens of thousands of people are gathered for the Democratic party events.  Police have also set up temporary jail cells to detain unruly protesters. 

Some information for this report was provided by AP.