KHARTOUM, 20 May (Reuters) - Sudanese police fired shots and arrested three men during an operation on Tuesday against suspected rebels involved in an attack on Khartoum, a security source said.

The incident took place in Omdurman, the suburb in western Khartoum that was attacked by Darfur rebels earlier this month, said a security officer who spoke on condition of anonymity.

Sudanese security services have been searching for rebels in the area since the attack, which killed more than 200 people.

Rebels from the Justice and Equality Movement (JEM) attacked Khartoum on May 10, the first time in decades of civil war that rebels from Sudan's peripheries had brought fighting to Khartoum's doorstep.

"Very early this morning, there was some firing from police who were looking for JEM soldiers," said the security officer. "They arrested three men."

There was a heavy security presence in central Khartoum and Omdurman on Tuesday. Heavily armed soldiers were seen on the streets around the United Nations headquarters, close to the capital's main airport.

Human rights groups have accused the government of carrying out mass arrests and torturing suspects.

The rebels were only stopped at the bridges over the Nile to the heart of Khartoum from Omdurman, a few kilometres from the army headquarters and the presidential palace.

Sudan's Minister of Defence has blamed Sudan's western neighbour Chad and other unnamed "international powers" of masterminding the attack -- a charge which Chad denies. (Reporting by Andrew Heavens; Editing by Ibon Villelabeitia) -- For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com

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