Sudan's President Omar al-Bashir is facing growing calls to step down following deadly protests over a dire economic crisis in the country.

Twenty-two Sudanese opposition political parties and groups on Tuesday demanded that Bashir transfer power to a "sovereign council" and a transitional government that would set a "suitable" date for democratic elections.

The groupings, calling themselves the National Front for Change, include some Islamist factions that were once allied with Bashir, who seized power in a 1989 military coup, as well as breakaway groups from large traditional parties.

"This government does not have the ability to overcome the economic crisis because the economic crisis is basically a political crisis," Mubarak Elfadel, chairman of the Umma Party, told reporters at a press conference in Sudan's capital, Khartoum.

"The government needs to end its rule and step down. We need to form a provisional council and a transitional government that will run this new stage and prepare us for new elections."

The groups said they will submit a memorandum comprising their demands to the president on Wednesday. They warned that failure to transition to a new political system would have "dire consequences" for Sudan.

The move came as a second party, Sudan Reform Now, announced it was withdrawing from the coalition government. 

It joins the Umma Party, which announced its withdrawal from the cabinet on Thursday, citing its support for the protesters and the government's failure to implement recommendations that were outlined in the national dialogue that preceded the coalition's formation.

'At crossroads'

Al Jazeera's Hiba Morgan, reporting from Khartoum, said the decision by Sudan Reform Now would likely weaken Bashir's government.

"Many opposition groups have come out to say they believe the protesters have legitimate reasons, but it is not clear yet if other parties will also follow in the footsteps of Sudan Reform Now and withdraw from the national coalition government."

She added: "Now Sudan is at a crossroads - people protesting and calling for Bashir to step down and opposition parties backing them, but the president and the ruling party remain defiant and determined to hold on to power."

The opposition front said the proposed transitional government would introduce freedoms, democracy and halt the ongoing strife in the western regions of Kordofan and Darfur, and the Blue Nile region south of Khartoum.

There was no immediate reaction from the government to the memorandum.

At least 19 people have been killed and hundreds wounded in protests that erupted in cities, including Khartoum, on December 19, after a government decision to increase the price of bread. The protests quickly developed into anti-government rallies demanding Bashir's resignation.

Human rights group Amnesty International has put the death toll at 37.

Earlier on Tuesday, Bashir ordered authorities to set up a committee to investigate violence during the anti-government protests.

In an address to the nation on Monday, he sought to defuse the anger sweeping the country, promising better days ahead and pledging more "transparency, effectiveness and justice in all our national institutions". 

"Our country is going through pressing economic conditions that have hurt a large segment of society," he said. "We appreciate this suffering, feel its impact and we thank our people for their beautiful patience."

The 2019 budget would maintain state subsidies on many commodities, raise wages, refrain from introducing new taxes and do more for the poorest. He did not elaborate.