The former speaker of the Sudanese parliament, Hassan al-Turabi, has formed a new political party, to be called the Popular National Congress.
Mr Turabi made the announcement at a fully-packed press conference in the Sudanese capital, Khartoum.
Many more people waited outside the press conference to show their support for Mr Turabi.
At least one cabinet minister is known to have resigned from the ruling party and joined Mr Turabi's new party.
Expulsion
Mr Turabi has a large and powerful Islamist following
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The former speaker's action followed his formal expulsion on Monday from the ruling National Congress Party, which he helped form and in which he served as secretary-general.
A new secretary-general of the party, Professor Ibrahim Ahmad Umar was also appointed during the weekend conference of the NCP, which was boycotted by Mr Turabi and more than 200 of his supporters.
President Bashir announced last month that the influential Dr Turabi was to be removed from his party post, in a deepening rivalry between the two most powerful persons in the Sudan.
Dr Turabi, an Islamist hardliner who helped the then General Bashir to assume power in 1989, has always been considered the real power behind the throne in Khartoum.
He was also considered the political and spiritual mentor of the president and the party.
Friends turned foes
Cracks in their relationship begun to become public in December last year when President Bashir declared a state of emergency and dissolved parliament which Dr Turabi headed.
The president accused his one-time mentor of trying to run a parallel administration and inciting elements in the Sudanese military to organise a coup against him.
The president's supporters also say Dr Turabi has been trying to mobilise students against the government.
Since the power struggle began, the Sudan Government has toned down its hardline Islamist stance and improved relations with many moderate Arab countries, notably its northern neighbour Egypt.
A BBC reporter in Khartoum, Alfred Taban, says that although the current power struggle with President Bashir has weakened Dr Turabi, he is far from a spent force because he attracts a considerable following - especially among Muslim fundamentalists - and his group of supporters includes some wealthy individuals.