Among those arrested were the Deputy Leader of the National Umma Party (NUP), Mariam al-Mahdi; NUP Secretary-General, Sara Naqdallah; the Political Secretary of the Communist Party, Mohamed al-Khatib and a senior member of the NUP, Um-Salama al-Mahdi said.
The arrests were made minutes before the start of a planned anti-government demonstration in the capital Khartoum, according to al-Mahdi.
The 12 opposition leaders were planning to lead a protest march “to the presidential palace to submit a memo’’ demanding President al-Bashir step down when they were arrested, he said.
He said when making the arrests, security forces and riot police fired tear gas at a crowd of hundreds of demonstrators, who had gathered near the Arab Market area in Khartoum.
The East African nation has seen ongoing protests for several weeks, since a sharp hike in bread and fuel prices caused public outcry.
In early January, the Interior Ministry said over 800 demonstrators had been arrested.
While the Sudanese government has confirmed the deaths of 29 protesters, human rights organisations say dozens of people have been killed in clashes with security forces.