"I expect the regime will fall before the 100 days finish," said Farouk Abu Issa, who represents more than 20 opposition parties.
He said the first month of the plan includes public forums, including at universities.
They would also ask the state security service for permission to hold a mass rally.
"If we don't get it, then there is another step we can take but we will not announce it now," he told a press conference.
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Issa said the opposition will also prepare "an initiative for democratic change," including for a transitional administration.
"We are going to send this initiative to the president," he said.
Last June and July, year scattered anti-regime protests sparked by inflation spread around the country.
They later petered out following a security clampdown.
A separate spurt of anti-government rallies occurred in December, when hundreds marched in Khartoum's streets after the death of four students.
The demonstrators called for the regime's downfall.
The periodic protests against Bashir's 24-year government, which calls itself Islamist, have failed to generate widespread following as did the Arab Spring revolts against authoritarian rulers in North Africa and the Middle East that began in December 2010.
Bashir took power in a 1989 coup that overthrew democratically elected prime minister Sadiq al-Mahdi, the Umma party leader.