Police arrested 250 anti-war protesters on Thursday shortly before John McCain accepted the Republican presidential nomination at the party's convention a short distance away.
The protest, dubbed "No Peace for the Warmakers," drew nearly 1,000 people who had planned to march to the downtown convention hall where the Republicans met.
Marchers chanted "Who is the terrorist? McCain is the terrorist" as they tried to cross several bridges that span Interstate 94 into downtown St Paul, Minnesota, but they were blocked by police.
After several standoffs, police in riot gear and many on horseback and bicycles ordered the protesters to disperse. When they did not after two hours, officers used flash grenades and tear gas to herd them onto a bridge.
"You are all under arrest," a police officer told the crowd.
The latest arrests followed 420 at several protests earlier in the week. Several journalists were among those arrested.
In a separate earlier incident on Thursday, roughly 60 people were arrested after they sat down in an intersection.
While more than 10,000 people have protested Republican policies peacefully during the four-day convention, they were overshadowed by several hundred who smashed shop windows and threw rocks and bottles at police on Monday.