* State news agency says 54 sent for trial
* Follows four days of clashes over anti-Islam film
* Egypt's Islamist president faces tough balancing act
By Patrick Werr and Tamim Elyan
CAIRO, Sept 15 (Reuters) - Riot police stormed into Cairo's Tahrir Square and rounded up hundreds of protesters early on Saturday after four days of clashes sparked by a film denigrating the Prophet Mohammad.
Security forces secured the square, just a few hundred yards from the U.S. embassy, and formed cordons in the surrounding roads. Plain-clothes officers patrolled the area, grabbing anyone they saw as suspicious. There was no sign of protests by mid-morning and traffic through Tahrir resumed.
In clashes overnight, a 35-year-old man died of birdshot wounds near the heavily-fortified embassy, the target of the protests, as police used volleys of teargas to repel a crowd throwing stones and petrol bombs.
The protesters said they wanted to expel the U.S. ambassador to punish Washington over the film which was produced in California. U.S. Defense Secretary Leon Panetta called on Egypt's government on Friday to ensure the embassy's safety.
Many of the men rounded up looked bruised and one was stripped down to his underwear. "Not so rough," shouted another as he was hustled away.
The square, the focus of last year's popular uprising that ousted President Hosni Mubarak, was strewn with garbage and a torched vehicle was towed away.
Egypt's state news agency said 220 "troublemakers and lawbreakers" were detained during the early-morning operation. It said 54 people were jailed for four days pending prosecution over the embassy clashes.
"Our presence here is to clear the square of people who are breaking the law," Interior Minister Ahmed Gamal el-Din said as he inspected the area. "We must preserve the square as a symbol of the revolution. That is the aim of our operation."
He said measures would be taken to ensure "those breaking the law" did not return.
More than 250 people have been reported injured in the clashes since Tuesday, when protesters climbed the embassy's walls and tore down an American flag in anger at the film.
The film portrayed the Prophet Mohammad as a womaniser and religious fake. U.S. Secretary of State Hillary Clinton has called it "disgusting and reprehensible".
Egypt's prestigious Al-Azhar mosque and seat of learning said it should be made a criminal offence around the world to "infringe on the symbols of Islam and of other world religions, after the attacks upon it that have disturbed world peace".
Mohamed Mursi, an Islamist and Egypt's first freely elected leader, has to strike a delicate balance, fulfilling a pledge to protect the embassy of a major aid donor while delivering a robust line against the film to satisfy his Islamist backers.
He has condemned the film, rejected violence and promised to protect diplomatic missions. His cabinet said Washington was not to blame for the film but urged the United States to take legal action against those insulting religion.
Islamists were among the crowd that scaled the embassy wall on Tuesday but many of the youths fighting in recent days were soccer fans with a long-standing grudge against the police.
Their motivation seemed a confused mixture of anger at the film, hostility to Mursi and the police, and soccer club loyalty.
In an interview with the BBC's Arabic service, Prime Minister Hisham Kandil said some of the 400 demonstrators arrested said they had received cash payments to protest at the U.S. embassy.
Those protesters were being interrogated to find out who was behind their actions, the BBC quoted him as saying. "The investigation will not end until the main mobiliser of these acts is uncovered," he said.
The security sweep was greeted with approval by many locals frustrated by the clashes, the worst in the capital since Mursi took office in June.
"It was very good what the police did. This was not a protest. It was thuggery," said Ahmed al-Tayyeb, a 60-year-old mosque imam.
Many Muslims regard any depiction of the Prophet Mohammad as blasphemous. The film has provoked outrage across the Middle East and led to the storming of several U.S. missions in the region. In the most serious incident in related violence, Washington's ambassador in Libya was killed.