Sudan's army Monday deployed troops around its headquarters in Khartoum and blocked several roads leading to the complex, where protesters have massed demanding President Omar al-Bashir resign, witnesses said.
Witnesses said soldiers were putting up barricades in streets near the compound, where thousands of protesters have been demonstrating outside since Saturday urging the army to back them.
Since protests erupted across Sudan in December security agents and riot police have cracked down on demonstrators, but the army has so far not intervened.
Several vehicles carrying NISS personnel and riot police arrived in the early hours Monday at the site where protesters have been demonstrating continuously since Saturday, witnesses told AFP.
"After that, security forces began firing tear gas at protesters," a witness said on condition of anonymity.
The gas was felt by residents in an upscale Khartoum district some five kilometres away from the army complex.
"I stepped out on my balcony hearing the sound of the gas canisters and could feel the gas in the air," said one resident.
A few hours later security personnel again fired tear gas at the protesters, witnesses said.
Protest organisers urged the residents of Khartoum and nearby areas to join the protesters who have been demonstrating for three days straight.
"Security forces of the regime are trying to disperse the sit-in by force," the organisers called the Alliance for Freedom and Change said in a statement.
"We call on all people around Khartoum to gather there to protect our people on the ground."
Thousands of protesters have been demonstrating since Saturday outside the army complex that also houses Bashir's residence and the defence ministry.
Chanting anti-government slogans, protesters have been urging the military to back them in demanding Bashir's resignation.
They accuse his administration of economic mismanagement that has led to soaring food prices and regular shortages of fuel and foreign currency.
The veteran leader has acknowledged that the economic concerns raised by the protesters were "legitimate".
Protests first erupted on December 19 after a government decision to triple the price of bread.
But they quickly morphed into nationwide demonstrations against Bashir's rule.
Officials say 32 people have died in protest-related violence so far, while Human Rights Watch has put the death toll at 51.
In recent weeks the scale and intensity of protests had dwindled due to a state of emergency imposed by Bashir, but Saturday saw a resurgence with thousands of protesters staging a continuous rally outside the army complex.
Protest organisers chose April 6 for the latest rally to mark the 1985 uprising that toppled the regime of then president Jaafar Nimeiri.