The contrasts were vivid: Pro-government supporters chanted "Death to America" and stomped on U.S. flags Wednesday while not far away, hundreds of opposition protesters denounced Iran''s leaders and appealed to America''s president to choose sides.

"Obama, Obama, you are either with them, or with us," the anti-government protesters chanted in Farsi, in an amateur video clip widely circulated on the Internet.

The new and startling appeal to President Barack Obama came as Iran''s opposition protesters returned to the streets in large numbers for the first time in nearly two months. Authorities were ready with the same sweeping measures they used to quell fierce election-fraud protests this summer and early fall: Sending paramilitary units to key locations to fire tear gas and beat people with batons.

Witnesses said they heard a man, apparently working for a paramilitary group, shout "Beat him up, beat him up," as he chased a protester in the crowd. "How is it possible to see such cruelty," another protester cried as he stood on the street, according to the same witnesses, who spoke on condition of anonymity because of fear of reprisals.