(Details, al-Arabiya bureau closed, quotes)
By Khalid Abdelaziz and Ulf Laessing
KHARTOUM, Sept 27 (Reuters) - Police fired teargas to disperse thousands of Sudanese demanding that President Omar Hassan al-Bashir step down on Friday, a day after deadly clashes with security forces whom rights groups accused of of shooting dead at least 50 people.
In the last few days, protests have drawn more than 5,000 people, the biggest for many years in Khartoum where Bashir - wanted by the International Criminal Court on genocide charges - has ruled since a coup in 1989.
About 3,000 people, angered by a police crackdown on demonstrations against the lifting of fuel subsidies, took to the streets after Friday prayers in Khartoum's twin city Omdurman, across the Nile, shouting "Freedom! Freedom!" and "The people want the fall of the regime!".
Defying a massive security presence, the crowd marched to the central market, holding up banners saying "No, no to price increases!"
Police fired teargas, sending some protesters running for cover. But most remained, some hurling stones at the police, others torching cars.
More than 2,000 people also demonstrated in Khartoum's northern Bahri district, a hot-spot for days of unrest, and other areas, witnesses said. Police again used teargas.
In Khartoum's centre, army trucks mounted with anti-aircraft guns, which are only usually deployed in strife-torn regions such as Darfur, were stationed in the street.
More than 100 soldiers, policemen and plain-clothes agents patrolled the government district on the banks of the River Nile.
Authorities also closed the bureau of al-Arabiya television station after complaining about its coverage of protests, the Dubai-based station said on its website.
The government has already put pressure on local newspapers only to use official statements when covering the demonstrations. At least two papers have stopped published in protest, editors said.
Sudanese officials dismissed the protests as insignificant.
"What happened today in Khartoum is limited. No more than 2,000 people took part in the protests," said Fateh Hassan al-Mahdi, spokesman for Bashir's National Congress Party. "This is insignificant compared to Khartoum's 7 million inhabitants."
Authorities arrested about 600 people suspected in violent riots and destruction of property, Interior Minister Ibrahim Mahmoud Hamad told state news agency SUNA. Trials for 100 "saboteurs" would start next week.
Protesters torched cars and petrol stations in Khartoum on Wednesday.
Police said late on Thursday that battles with protesters had killed 29 people, among them police officers. Sudanese opposition activists have put the death toll at over 100.
But London-based Amnesty International and the New York-based African Center for Justice and Peace Studies said at least 50 people had been killed by gun shots to the chest or head, citing witnesses, relatives, doctors and journalists.
Among the dead was a 14-year-old boy, while most other victims seemed to be between 19 and 26 years old, the groups said in a statement. Hundreds had been detained, it said.
"Shooting to kill - including by aiming at protesters' chests and heads - is a blatant violation of the right to life, and Sudan must immediately end this violent repression by its security forces," said Lucy Freeman, Africa Deputy Director at Amnesty International.
Sudanese officials could not be immediately reached for comment but Information Minister Ahmed Belal Osman said late on Thursday any figures higher than 29 were inaccurate.
New York-based Human Rights Watch (HRW) said in a separate statement it had confirmed that the death toll was higher than the official 29. It did not give a number.
"The police and national security forces fired teargas, rubber bullets, and according to credible reports, live ammunition into the crowds," HRW said.
It was not immediately possible to verify the allegations.
Bashir still enjoys support from the army, security apparatus, his ruling party and wealthy Sudanese with extensive business interests. (Writing by Ulf Laessing; Editing by Ralph Boulton and Robin Pomeroy)