Protesters are facing a “massacre in a treacherous attempt to disperse the protest”, the main protest group said in a statement.
Two people have been killed and dozens injured, a medical association linked to the protesters said, as explosions and gunfire were reported.
Roads are said to have been blocked in what may be an attempt to move against the protest which has been going on for weeks.
The AP news agency said smoke was rising from the area and civilians were being detained.
Protest leaders urged supporters to converge on the area.
Sudanese protesters cook in the street in Khartoum
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.
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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