* Anger over corruption and cutting of subsidies
* Biggest protests for years in Khartoum and twin city
* Interior minister says about 600 arrested
By Khalid Abdelaziz and Ulf Laessing
KHARTOUM, Sept 27 (Reuters) - Police fired teargas on Friday to disperse thousands of Sudanese demanding that President Omar Hassan al-Bashir step down, a day after clashes in which rights groups accused security forces of shooting dead at least 50 people.
Bashir, who seized power in a 1989 coup, has been spared the sort of Arab Spring uprising that unseated autocratic rulers from Tunisia to Yemen since 2011, but anger has risen over corruption and rising inflation in the vast African country.
On Friday more than 5,000 people demonstrated after Muslim prayers in the biggest protests for many years in the Khartoum area.
Angered by a police crackdown on demonstrations against the slashing of fuel subsidies, about 3,000 took to the streets in Khartoum's twin city Omdurman, across the Nile, shouting "Freedom! Freedom!" and "The people want the fall of the regime!"
Defying a heavy 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 and others torching cars.
More than 2,000 people demonstrated in Khartoum's northern Bahri district, a hot spot for days, and in other areas, witnesses said. Police again used teargas.
Bashir, has remained in power for almost 25 years despite armed rebellions, U.S. trade sanctions, an economic crisis, an attempted coup last year and an indictment from the International Criminal Court for war crimes in the western Darfur region. He still enjoys support from the army, his ruling party and wealthy Sudanese with extensive business interests.
In a written statement, the United States condemned the crackdown, accused the Sudanese authorities of using excessive force and expressed alarm at reports that the government had arrested or detained activists and restricted access to the Internet and mobile telephone networks.
"The United States condemns the Government of Sudan's brutal crackdown on protesters in Khartoum, including the excessive use of force against civilians that has reportedly resulted in dozens of casualties," said State Department spokeswoman Jen Psaki.
"Such a heavy-handed approach by Sudanese security forces is disproportionate, deeply concerning, and risks escalation of the unrest."
Sudan summoned the U.S. charge d'affaires in Khartoum to protest against the United States' failure to issue Bashir a visa to attend the U.N. General Assembly taking place this week in New York, the Foreign Ministry said.
In Khartoum's centre, army trucks mounted with anti-aircraft guns, usually only 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 Nile.
Authorities closed the bureau of Al Arabiya television station after complaining about its coverage of protests, the Dubai-based station said on its website.
They also shut down the office of Abu Dhabi-based news channel Sky News Arabia, banned its correspondent from working and confiscated its equipment, the station said.
The government has 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 of violent riots and vandalism, Interior Minister Ibrahim Mahmoud Hamad told state news agency SUNA. Trials for 100 "saboteurs" would start next week.
Protesters had torched cars and petrol stations in Khartoum on Wednesday. Opposition activists have blamed government agents for some of the damage.
The latest round of unrest began on Monday after the government announced another set of fuel subsidy cutbacks, causing pump prices to almost double overnight.
The cuts have been driven by a severe financial crunch since the secession of oil-producing South Sudan in 2011, which deprived Khartoum of three-quarters of the crude output it relied on for state revenues and food imports.
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.
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.