The agencies said security forces surrounded the party's headquarters on Wednesday morning, to stop a rally to celebrate the 20th anniversary of the fall of a previous regime. Scores of officials and party members were arrested.

Subsequently, officials received a notification banning all political activities by the party, claiming it had not been properly registered.

"All of these problems have come about because the Ummah Party's position is that we should cooperate with international resolutions," Abdel Nabi Ali Ahmed, the party's secretary-general, was quoted by Reuters news agency as saying.

Ummah's leader, Sadiq al-Mahdi, had on Friday expressed support for a UN resolution calling for those implicated in crimes in Darfur to be tried by the International Criminal Court.

"The Ummah Party is being targeted for their public support of the ICC resolution," a Sudanese official, who declined to be named, told IRIN on Thursday.

Sudan's Council of Ministers declared its "total rejection" of the resolution on Sunday, while President Umar al-Bashir said his government would refuse to hand over any Sudanese citizens to be tried outside the country.

On Tuesday, tens of thousands of demonstrators took to the streets in Khartoum, to support their government's rejection of the resolution.

After being told it could not hold its rally in a public place, the Ummah Party decided to move Wednesday's event to its headquarters. But the situation escalated when armed police prevented people from entering the compound.

The police then went into the compound themselves, and fired tear gas at members who had gathered outside.

Among those arrested were the party's secretary-general, Ahmed; the deputy secretary-general, Isma'il Adam; the head of the party's Omdurman branch, Umar al-Shahid; and a member of the party's political bureau, Sadiq al-Fadhil, according to a statement on the opposition Democratic Unionist Party website on Thursday.

Ummah's rally was intended to mark the anniversary of the end of Jaafer Nimeiri's one-party regime in 1985, which was overthrown after days of street protests. Al-Mahdi became prime minister during a brief period of multi-party democracy, but al-Bashir overthrew his government in a coup in 1989.

Sudan's current government also banned the opposition Popular Congress of Hassan al-Turabi in 2004. Most opposition parties, apart from Ummah, operate outside the country.

However, they are gradually returning to Khartoum following the signing of a comprehensive peace agreement between the Sudanese government and the southern Sudan People's Liberation Movement/Army (SPLM/A) on 9 January.