Sudanese security forces in pickup trucks have opened fire on thousands of mourners marching to the funeral of protester Salah al-Sanhouri.

Protesters said dozens of pickup trucks and security forces surrounded them in a main street of the capital Khartoum before firing tear gas and live ammunition.

"The cars came from the back and the front while we were marching in the street," a woman protester said.

"The tear gas was very strong. The people fled trying to escape, taking shelter inside homes," she added.

Earlier in the day, women had blocked a side-street to prevent police from deploying at the funeral of the 26-year-old pharmacist.

Mr Sanhouri's family says he was shot outside his pharmacy as a march passed on Friday.

The violent crackdown that aimed to quash the biggest street demonstrations in two decades could now actually be propelling them, activists said.

"The excessive use of force means that the regime is becoming bare of any political cover and it is declaring war against its own people," said Khaled Omar, a member of the Change Now youth movement, one of the groups calling for protests.

"This will backfire inside the regime itself and cause cracks within and lead to its collapse," he warned.

In the first sign of significant resistance within the regime, 31 politicians, including members of President Omar al-Bashir's ruling party and military officials signed a petition on Saturday calling on the president to carry out reforms.

Among them were ruling National Congress Party leading member Hassan Ali Rizk and Ghazi Salah Eddin, a former information minister and a presidential adviser.

The petition called for a reversal of austerity measures, the creation of a mechanism for national consensus and an investigation into the killings.

Among the signatories were members of the Islamic Movement, which has hitherto supported the ruling party.

Protesters say that austerity measures hit the poor particularly hard but leave intact a corrupt system where senior officials grow wealthy.

"This is a government of thieves who looted the country and starved us," the slain pharmacist's uncle said.