Khartoum - Sudanese forces entered the site of a sit-in in central Khartoum early on Monday amid gunfire, witnesses and Arab television stations said, in what activists described as an attempt to disperse protesters demanding civilian rule.
A medical association affiliated to protesters said at least two people were killed and dozens injured in the raid, which was still in progress.
"The protesters holding a sit-in in front of the army general command are facing a massacre in a treacherous attempt to disperse the protest," the main protest group said in a statement, urging the Sudanese people to come to their aid.
The violence comes amid a persistent deadlock in talks between protesters and Sudan's military rulers over demands to hand over power to civilians.
The Transitional Military Council (TMC), which assumed power in April when the military ousted President Omar al-Bashir after three decades in office, has offered to let protesters form a government to run the country but insists on maintaining overall authority during an interim period.
Demonstrators want civilians to run the transitional period and lead the North African country of 40 million to democracy.
Stability in Sudan is crucial for a region grappling with violence that stretches from the Horn of Africa to Libya.
The TMC has repeatedly said it would not use force to disperse the protesters, often comprising thousands of young men and women who take turns camping outside the Defence Ministry.
No statement about the violence has yet been issued by the TMC.
The British Ambassador in Khartoum said in a message on his Twitter account he was "extremely concerned by the heavy gunfire I've been hearing over the last hour from my Residence and reports that Sudanese security forces are attacking the protest sit-in site resulting in casualties".
"No excuse for any such attack. This. Must. Stop. Now," he wrote.
Live footage broadcast by Arab television stations showed chaotic scenes, with protesters running away as black smoke rose from tents apparently torched by the raiding force.
A Reuters witness saw troops wielding batons deploy in central Khartoum and close roads, apparently to try to block people from reaching the protest site.
Nile bridges that connect various parts of the Sudanese capital have also been blocked.
The sit-in had become the focal point of protests that started in December, sparked by a severe financial crisis that caused cash shortages and bread price hikes.
Sudan, one of the largest countries in Africa, has long been on a US list of countries that support terrorism, which has hampered foreign investments.Reuters