Protesters have been staging a sit-in camp for weeks, demanding an end to military rule in Sudan.
Troops have entered a sit-in protest in the Sudanese capital, Khartoum, following weeks of protests against military rule.
The British ambassador in Khartoum, Irfan Siddiq, said he was "extremely concerned by the heavy gunfire I've been hearing from my residence".
There were "reports that Sudanese security forces are attacking the protest sit-in site resulting in casualties", he added.
Protest leaders said at least five people had been killed, while the Sudan Doctors' Committee said a large number had been wounded.
The ruling military council said it had been targeting what it considers to be a problematic area near the sit-in, nicknamed "Colombia."
Spokesman Shams al-Deen al-Kabashi said a number of people from that area then moved to the main protest site.
Sudanese forces around Khartoum's army headquarters
Activist Dura Gambo said large numbers of troops entered the protest area and arrested people trying to leave.
"They have used the heavy rain yesterday and moved in the early morning to disperse people," she said.
Live footage broadcast by Arab TV stations showed protesters running away as black smoke rose from tents which appeared to have been set on fire.
Roads are said to have been blocked by troops wielding batons.
In Omdurman, Khartoum's twin city, there were reports of protesters blocking roads with stones and burning tyres.
Demonstrators are facing a "massacre in a treacherous attempt to disperse the protest", the main protest group said in a statement, urging supporters to converge on the area.
A leader of the Declaration of Freedom and Change Forces, Khalid Omar Yousef, said protests would be escalated, including "full civil disobedience".
Mr Siddiq said there was "no excuse for any such attack".
"This. Must. Stop. Now," he tweeted.
Sudan's long term ruler, Omar al Bashir, was overthrown by the military in April following mass protests against his 30-year rule.
Since then there has been a Transitional Military Council (TMC).
It is proposed that a sovereign council lead Sudan for three years as it embraces civilian rule.
But there has been persistent deadlock in talks between the protesters and the military.
The demonstrators want limited military representation on the sovereign council, but the generals want overall control during the interim period.
The TMC has said repeatedly that it will not use force to disperse the protesters.
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