At least eight people were reported wounded by gunshots near a Khartoum sit-in Wednesday, before crucial talks between military chiefs and protest leaders over a transitional governing body.
Army generals and protest leaders are expected to finalise the make-up of a new body to govern Sudan for three years, the thorniest issue in installing civilian rule.
But just hours before the talks were due to start, a spokesman for the umbrella protest group, the Alliance for Freedom and Change, wrote on Facebook that eight people had been wounded by live fire.
A witness told AFP that gunshots had been fired near the protest camp outside the army headquarters in central Khartoum.
The British ambassdor to Khartoum said Sudanese security forces had fired at protesters.
"Extremely concerned by use of live ammunition by Sudanese security forces against protesters in Khartoum today, with reports of civilian casualties," Irfan Siddiq wrote.
"Military council must act to stop this now. No more excuses."
Protest leaders responded by urging people to boost the numbers at the demonstration, while avoiding clashes.
Security forces were later seen chasing protesters in downtown Khartoum and removing some roadblocks that demonstrators had put up, an AFP correspondent said.
The protest movement that brought down president Omar al-Bashir after 30 years of iron-fisted rule is demanding a civilian-led transition, which the generals have steadfastly resisted since bowing to their demands and toppling the autocrat.
The latest breakthrough came despite the talks being marred by violence that left six people dead on Monday at the sit-in outside the army headquarters.
Call to support demo
After Wednesday's incidents, a key group in the protest movement urged people to join the thousands of demonstrators at the site, some of whom have camped out round-the-clock for weeks.
"We call on everybody to join the sit-in immediately and support the protesters," the Sudanese Professionals Association, a group of doctors, engineers and teachers, said in a statement.
It added: "We are calling on revolutionaries to restrain themselves, be calm and peaceful and avoid any confrontation or clash with any group whatever the circumstances."
Wednesday's crucial negotiations are due to start at 21:00, said Khalid Omar Yousef, a leader from the Alliance for Freedom and Change.
"The announcement is expected after midnight," he told AFP.
The current talks began on Monday and the two sides have since agreed on an overall civilian structure, including a three-year transitional period for the full transfer of power to a civilian administration.
They have also agreed that parliament be composed of 300 members for the transition, with 67% from the alliance and the rest drawn from other political groups.
The first six months of the transition would be devoted to reaching peace accords with rebels in war zones including Darfur, Blue Nile and South Kordofan.
The United Arab Emirates, widely seen as backing the ruling generals, hailed the agreement on a transitional period.
It "puts Sudan on the road of stability and recovery after years of Bashir and (Muslim) Brotherhood's dictatorship," its minister of state for foreign affairs, Anwar Gargash, tweeted.
Deal within hours
The UAE and Saudi Arabia have offered $3 billion in aid for Sudan.
The composition of the new sovereign council has been the toughest part of the negotiations, with the two sides so far proposing different compositions of the body which is expected to take all key decisions concerning national issues.
The generals want it to be military-led, while the protesters insist on a majority civilian body.
General Yasser al-Atta, one of the members of the current ruling military council, vowed to reach a deal by early Thursday that "meets the people's aspirations".
The new council is expected to form a transitional civilian government, which would then prepare for the first post-Bashir election after the three-year changeover period ends.
Protest leader Yousef downplayed the role of the proposed ruling council, insisting Sudan would have a powerful cabinet.
"All powers will be in the cabinet's hand, which will be formed by the Alliance for Freedom and Change," he said.
On the defence and interior ministries would be headed by military figures, he said.
Tensions have soared since Monday's shootings, which the United States blamed on security forces.
The United States has consistently called on the military council to transfer power to civilians.