SOKOTO, Nigeria (Reuters) - Nigerian police rounded up suspects on Saturday after hundreds of Sunni Muslim militants burned a local government office in the far northwestern city of Sokoto, a government spokesman said.

Hundreds of soldiers and policemen were guarding sensitive locations in the tense city on the edge of the Sahara desert, where politically charged fighting between Sunnis and Shi'ites has killed about a dozen people over the past two months.

"I was told that nine people directly connected with the arson attack (on Friday night) were arrested at the site, and there have been a lot more arrests this morning," Sokoto State government spokesman Mustapha Shehu said.

He did not have a specific number of arrests and police could not immediately be contacted.

"Lots of security measures have been taken. There is a heavy presence of armed police and soldiers in all strategic places all over Sokoto now," Shehu said.

The government had deployed about 500 extra troops and riot police on Friday to guard key areas of the city during the Muslim day of prayer, but the reinforcements failed to deter Sunni activists from burning the government office.

The militants, enraged that their leader was arrested for instigating violence earlier in the week, also torched 25 cars in the government compound on Friday night and tried to set fire to a radio station, although troops stopped them.

The arson attacks came a day after Sunni militants armed with machetes killed a Shi'ite man at his home, prompting the region's religious authority, the Sultan of Sokoto, to appeal to Sunni and Shi'ite leaders to help quell the violence.

Religious, ethnic and communal conflicts have killed more than 20,000 Nigerians since the nation of 140 million people returned to democracy six years ago. Nigeria is split about evenly between Christians in the south and Muslims in the north.

In Sokoto, the dispute began over doctrinal differences between Shi'ites and Sunnis and access to the central mosque, but State Governor Attahiru Bafarawa has said politically motivated militants were stoking the conflict.

Bafarawa, who belongs to an opposition party, says members of Nigeria's ruling People's Democratic Party have instigated violence and he has written to President Olusegun Obasanjo to complain.

The Sunni governor's stance that Shi'ites should have access to Sokoto's central mosque has made him unpopular with some Sunnis.

A letter has been circulating in Sokoto since Monday in which Shi'ites are threatened with attacks unless they leave town or renounce their faith at the central mosque. Worshippers say at least 200 Shi'ites have done so.