Crowds waved flags and chanted slogans, warning each other to keep a look out for the security forces who have launched deadly crackdowns on previous rallies, leaving at least 79 people killed and hundreds wounded, according to independent medics.

"Don't show your back to the military, they are not to be trusted," shouted protesters in the city of Omdurman, the twin city of the capital Khartoum, lying across the Nile river.

"Give your back to the streets, it will not betray you," they added.

Regular mass protests have rocked the troubled northeast African nation since the October 25 military takeover led by army chief Abdel Fattah al-Burhan.

The power grab derailed a fragile power-sharing arrangement between the army and civilians that had been painstakingly negotiated after the 2019 ouster of longtime autocrat Omar al-Bashir.

Demonstrations on Thursday also took place in the south of Khartoum, where protesters blocked the streets with stone and brick barricades, an AFP correspondent said.

Arrests condemned

In central Khartoum, security forces fired tear gas at dozens of protesters to stop them from heading toward the presidential palace.

As the protests thinned out by the evening, security forces again fired tear gas to disperse the demonstrators in Omdurman and east Khartoum, witnesses said.

Many were seen carrying Sudanese flags and holding up posters of fellow protesters who have been killed, with activists saying Thursday's rallies were only a warm-up for a larger protest they plan on February 14.

"Today's protests are in preparation for the mass demonstrations on Monday," activist Roaa Bashir said.

Hundreds of people, including political leaders, activists and journalists have been arrested since the power grab, and Sudan's prominent Umma party has demanded an independent investigation into reports of "violations against peaceful protesters".

On Wednesday, security forces arrested two leading figures from the opposition Forces for Freedom and Change (FFC), the main civilian bloc which was ousted after the power grab.

The arrests come a day after the two men -- ex-minister Khaled Omar Youssef and FFC spokesman Wagdi Saleh -- joined an FFC delegation for talks with UN special representative Volker Perthes, as part of efforts launched last month hoped to resolve the deepening crisis.

The international community have called for the immediate release of the FFC figures as they slammed the broadening crackdown.

Norwegian ambassador Therese Loken Gheziel said the arrests "undermines efforts to resolve the crisis", while British ambassador Giles Lever said it showed "a lack of good faith".