CAIRO — The police used tear gas and fired in the air Tuesday to disperse thousands of protesters trying to march on the presidential palace to demand that Omar Hassan al-Bashir, Sudan’s president of 29 years, step down, according to activists and video clips posted online.

The clips purported to show crowds of several hundred each gathering on side roads and headed toward the palace on the bank of the Blue Nile. They sang patriotic songs and chanted “freedom,” “peaceful, peaceful against the thieves” and “the people want to bring down the regime.”

One clip showed the lifeless body of a protester in Khartoum being carried away and placed inside a car that drove away. The protester’s head showed a gaping wound, and the voice of another protester could be heard saying he was deliberately shot by a sniper. Earlier images circulated by activists showed police snipers on rooftops near the palace ahead of the march.