Heavy gunfire has been heard outside the army headquarters in the Sudanese capital Khartoum where thousands of protesters have been holding a sit-in for the third night in a row.
They are calling for the resignation of President Omar al-Bashir.
Witnesses reported seeing people running for cover after the shooting began. Earlier, tear-gas was fired.
It appears to be the latest attempt by government security agents to break up the protests.
One of the protesters, Ahmed Mahmoud, told the BBC that "tear-gas and live bullets were used" by National Intelligence and Security Service (NISS) agents against protesters.
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He added that army soldiers had provided sanctuary for protesters within their compound.
"It is pointless for Omar Bashir to continue using his thugs to get us off the streets as we are not going anywhere."
At least two soldiers are reported to have died since the demonstrations outside the army HQ began on Saturday.
Previous attempts to break up the crowds have also led to reports of soldiers intervening to protect protesters from NISS agents.
There have been international calls urging the government to refrain from using force against civilians. President Bashir has called for talks to end the crisis.
The country's interior minister said on Monday that seven protesters had been killed and 15 injured, while 42 members of the security forces had been injured. He added that almost 2,500 people had been arrested.
Why are they protesting?
Protests against Mr Bashir, who has governed Sudan since 1989, have been under way for several months.
The protests were originally sparked by a rise in the cost of living, but demonstrators are now calling for the president to resign.
Over the weekend, a large group gathered outside the Khartoum headquarters of the army, defence ministry and the president.
The protesters want the armed forces to withdraw their support for the government. Representatives of the protesters say they are seeking talks with the army regarding the formation of a transitional government.
Omar el-Digeir, a senior protest member, told AFP news agency the group were seeking a path "that represents the wish of the revolution".
Monday marked the third night of the sit
The government has been criticised by rights groups for a heavy-handed response to the unrest.
What has the response been?
Dramatic video emerged on Monday showing soldiers firing at an unclear target as civilians took cover behind them. The protesters said the soldiers were responding to gunfire from NISS agents.
Other eyewitnesses have alleged the military fired warning shots while chasing the agents off.
Information Minister Hassan Ismail has contradicted the reports about the divide.
"The security apparatus are coherent together and working with positive energy and in harmony," he said on Monday.
United Nations Secretary-General António Guterres has urged all parties to "exercise utmost restraint and avoid violence".
Government officials now admit 38 people have died since the unrest began in December, but the pressure group Human Rights Watch says the number is higher.
Why is the president controversial?
Mr Bashir's rule has been blighted by accusations of human rights abuses.
He is subject to an International Criminal Court (ICC) arrest warrant over accusations of of genocide, war crimes and crimes against humanity.
The US imposed sanctions against the country more than 20 years ago, accusing Khartoum of sponsoring terror groups.
Last year the Sudanese pound fell rapidly in value and inflation rose. The government then announced the price of fuel and bread would rise, sparking protests.
In February, it looked as though the president might step down, but instead Mr Bashir declared a state of national emergency.
The latest protests mark the 34th anniversary of the coup that overthrew the regime of former President Jaafar Nimeiri.