Police in Sudan used tear gas and stun grenades to disperse thousands of protesters in the capital Khartoum on Monday.
Demonstrators had gathered to demand an immediate end to military rule and renounce a deal between civilian politicians and the ruling generals.
Monday's protests also marked the fourth anniversary of the start of an uprising that toppled dictator Omar Al Bashir's 29-year regime.
“The street is where the solution is,” said Mustafa Abdullah, a Khartoum protester from the powerful Resistance Committees — a pro-democracy, neighbourhood-based group.
“We will keep doing this even if it takes years.”
Authorities declared Monday a holiday and closed several Nile bridges connecting the capital Khartoum with its twin cities of Bahri and Omdurman — a tactic often used to prevent protesters from gathering and potentially overrunning police lines.
They also sealed off roads leading to the armed forces' headquarters and the Defence Ministry.
The protesters, many of whom were waving Sudanese flags and images of fallen comrades, called for civilian rule and justice for deaths that occurred during past rallies.
They marched to within about 1.5km of the Nile-side presidential palace in central Khartoum before armoured vehicles blocked their way. Police later chased them through the streets.
In addition to the heavy use of tear gas and stun grenades, police vehicles also sprayed protesters with water.
There were reports of similar protests in other Sudanese cities.
Monday's protests were called by the Resistance Committees, which is opposed to a preliminary deal reached on December 5 between the military and a major civilian coalition.
The agreement, in theory, would restart Sudan's democratic transition, leading to military generals quitting politics and a civilian-led government that would steer the country for 24 months until elections.
“I hope that the political process will realise the demands and aspirations of the Sudanese men and women who took to the streets four years ago,” UN Special Representative in Sudan Volker Perthes tweeted on Monday.
Mr Perthes has played a key mediating role over the months of negotiations that produced the December 5 deal.
The military, led by Gen Abdel Fattah Al Burhan, seized power in October last year, upending the transition that followed the 2019 ousting of Al Bashir. The power grab also plunged Sudan deeper into economic crisis and unleashed a wave of street protests in which at least 122 people have been killed by security forces, with another 6,000 injured.
“Our struggle continues to bring down the military and our resolve to do so does not soften,” said a statement by the Resistance Committees.
“We remain united over the principles of no negotiations, partnership or bargaining with the murderers.”
Sudan's Finance Minister Gibril Ibrahim, a former rebel leader who did not sign the agreement, called the document “exclusionary”.
“Today we observe the fourth anniversary of the glorious December revolution which did not achieve its goals,” he said on Monday on Twitter.
“The country is in dire need of a national consensus that does not exclude anyone.”
Abdelrahman Ali, a Khartoum protester, echoed the minister's rejection.
“It's a reinvention of the situation where the military dominates,” he said, dismissing the civilian politicians who signed the deal as “totally divorced from the Sudanese street”.