(CNN) Some members of the Sudanese army have moved to protect protesters in the capital, Khartoum, after national security forces attempted to break up the third day of a mass sit-in outside the presidential compound and army headquarters, according to an opposition union of doctors.

A soldier was killed in clashes on Monday as he tried to defend demonstrators from security agents who were firing tear gas and live ammunition to disperse the crowds, the Central Committee of Sudan Doctors (CCSD) said.

Signs of a split between Sudan's army and its security service could mark a significant turning point for protesters, who have been calling for the removal of President Omar al-Bashir since December.

A protester holds bullet cartridges as others gather in front of security forces Monday outside the military headquarters.

Over the weekend, crowds converged on the heart of Bashir's rule -- Sudan's presidential palace and the nation's military headquarters -- responding to a renewed call for protests from the Sudanese Professionals Association (SPA), an umbrella organization of doctors, lawyers and journalists that has led many of the demonstrations.

The sit-in has grown into one of the biggest rallies since the uprising began earlier this year, with crowds braving temperatures of more than 110 degrees Fahrenheit (43 degrees Celsius) to camp out in the streets of the capital.

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