Sudan has summoned seven leaders of the embattled main opposition party, the SPLM-North, after closing all its offices and suspending its activities, a senior member said on Sunday.

Security forces summoned the party officials -- from Al-Qadarif, Sennar and Northern states and from the war-torn region of Darfur in western Sudan -- on Saturday evening, Muatassim Hakim told AFP.

"The authorities ordered them last night to come to the security offices at 9 o'clock (0600 GMT) this morning. They are still under investigation," he said.

The Khartoum government appears increasingly determined to assert its authority within its new borders following South Sudan's independence on 9 July, moving to disarm troops outside its control.

On Saturday it shut down all the offices of the SPLM-North, charging it was not a legally represented political party, according to Hakim and another source within the party.

The move came a day after fighting erupted in the Blue Nile state between the army and ex-rebel troops loyal to the elected governor, Malik Agar, the SPLM-North's chairman, who President Omar Al-Bashir sacked from his job on Friday after declaring a state of emergency.

The SPLM-North's Secretary General Yasser Arman vowed on Saturday to fight for regime change through armed struggle and mass protests.

The "coup" against Agar in Blue Nile state demonstrated that constitutional change in Sudan was impossible under the present government, he charged.

Arman said he and the party's vice chairman had met on Friday with leaders of the three main Darfuri rebel groups to form the nucleus of a united political and military movement for change.