An umbrella of Sudanese independent professional unions is calling on people to march on the presidential palace in the capital, Khartoum, to demand autocratic President Omar Bashir step down.

The umbrella, which enjoys the support of opposition parties, says the march will take place on Thursday.

Previous marches to the palace witnessed clashes with police who used tear gas and batons to disperse protesters.

Sudan's main opposition groups appeared in a joint press conference on Wednesday, their first since this wave of unrest erupted in December to call for an end to al-Bashir three-decade rule.

They called for a four-year transitional government followed by elections.

The protests in Sudan began initially over surging prices and a failing economy, but quickly turned into calls for al-Bashir's resignation.