Denver, Colorado - After days of low-key protests, 100 demonstrators were arrested by police on the sidelines of the Democratic National Convention after refusing requests to disperse, police media officials said Tuesday.

Police said in a statement that a protest crowd of about 300 people had gathered near Civic Center Park near the state capitol building in downtown Denver, and "many were observed carrying rocks and other items that could be used to threaten public safety."

Denver police used pepper spray after the protestors "suddenly rushed a police safety line," police said.

Those arrested were charged with obstructing streets and disobeying lawful order, officials said.

The Democratic Party's presidential nominating convention opened Monday evening in Denver, Colorado. On Thursday, US Senator Barack Obama is scheduled to deliver an acceptance speech at a football stadium to a crowd of 76,000, becoming the first African-American to win a major-party presidential nomination.

Federal, state and local police are a strong, visible presence throughout Denver, with police dressed in riot gear on many street corners or riding on pickup trucks that rush to possible threat scenes. Horse-mounted troops are also a presence.

In a separate, possibly more serious incident, federal police have arrested three men in connection with what authorities believe could have been a threat against presidential candidate Barack Obama, officials said. More details will be released later Tuesday.

The incident with the protestors occurred about 7:15 pm Monday.

The Civic Center park has been set aside for protestors, but they have kept a relatively low profile so far. The anticipated thousands of demonstrators have not appeared to date.

"In order to protect the public when the crowd surged forward, two officers deployed their pepper spray and one officer used a pepper ball device," the police statement said.

Pepper balls are similar to a paint ball containing the same material as pepper spray.

Those arrested could post bond or be taken for a first appearnace in Denver County Court, police said.

Police and rescue helicopters have been periodically running practice missions over Denver for weeks in preparation for the convention, and local residents say some of the aircraft have police perched in the open doors with guns at the ready.