* One person killed in clashes, 21 treated in hospital
* Police fire teargas to disperse crowds
* Clashes diminish as day ends
Supporters and opponents of ousted Egyptian President Mohamed Mursi clashed in central Cairo on Monday, hurling stones and firing birdshot and fireworks at each other, witnesses said.
The state news agency said one person was killed from a gun shot to the chest, while 21 people needed hospital treatment, the head of the paramedics said.
Blood stains and broken glass littered the pavement between the duelling sides, and injured people were whisked away from the clashes on motorbikes in the worst political violence in Cairo since July 16, when seven died in confrontations.
The state news agency said 20 "troublemakers" were arrested, without giving details if they were supporters or opponents of the ousted president.
Police fired teargas to break up the crowds and Mursi's foes said the security forces were clearly on their side.
"The police are with us. Those fascists attacked us again. May God punish them," said Samir Hafiz, 22, with cuts on his hands and face, hurling rocks at the pro-Mursi activists from behind the police line.
Witnesses said hundreds of young men clashed as backers of Mursi's Muslim Brotherhood movement appeared to try to march on Tahrir Square, epicentre of mass demonstrations which led Egypt's army to oust the elected Islamist leader on July 3.
"They fired on us with birdshot and pistols. They tried to overrun the square." said Tarik Sabir, 41, an employee in a petrol company, who was wounded in the thigh by birdshot.
As dusk fell, the clashes faded and Mursi's opponents returned to their sit-in at Tahrir, blowing whistles triumphantly and chanting "We saved the square!"
Tahrir Square has been a focal point for demonstrations since mass protests there led to the downfall of U.S.-backed strongman Hosni Mubarak in February 2011.
Two people were killed in separate clashes between pro and anti-Mursi protesters in the Nile Delta city of Kalubia, a Health Ministry official told Reuters.
Around 100 people have died in violence since the downfall of Mursi earlier this month - most of them Muslim Brotherhood supporters.
Violence has also wracked Egypt's Sinai Peninsula this month. One civilian was killed and four security personnel were injured in separate attacks by militants against army check points, security sources said on Monday.
The attacks come the day after militants, angered by the downfall of Mursi, killed six Egyptians, including two civilians, in the Sinai, which borders Israel and Gaza.