Six protesters were shot and killed in Sudan on Thursday, medics said, as large crowds took to the streets despite heavy security and a communications blackout to rally against the military leadership that seized power eight months ago.

In central Khartoum, security forces fired tear gas and water cannon as they tried to prevent swelling crowds from marching toward the presidential palace, witnesses said.

They estimated the crowds in Khartoum and its twin cities of Omdurman and Bahri to be at least in the tens of thousands, the largest this year. In Omdurman, witnesses reported tear gas and gunfire as security forces prevented protesters from crossing into Khartoum.

The protests in the capital and other cities marked the third anniversary of huge demonstrations during the uprising that overthrew longtime autocratic ruler Omar al-Bashir and led to a power-sharing arrangement between civilian groups and the military.

Last October the military, led by General Abdel Fattah al-Burhan, toppled the transitional government, triggering rallies demanding the army quit politics.

Some of Thursday's protesters carried banners calling for justice for those killed in previous demonstrations. Others chanted, "Burhan, Burhan, back to the barracks and hand over your companies," — a reference to the military's economic holdings.

Earlier, protesters barricaded some of Khartoum's main thoroughfares with stones and burning tires.

June 30 also marks the day Bashir took power in a coup in 1989.

"Either we get to the presidential palace and remove al-Burhan or we won't return home," said a 21-year-old female student protesting in Bahri.

It was the first time in months of protests against the October coup that internet and phone services had been cut. After the military takeover, extended internet blackouts were imposed in an apparent effort to weaken the protest movement.

Staff at Sudan's two private sector telecom companies, speaking on condition of anonymity, said authorities had ordered them to shut down the internet once again on Thursday.

Bridges closed

Phone calls within Sudan were also cut, and security forces closed bridges over the Nile linking Khartoum, Omdurman and Bahri — another step typically taken on big protest days to limit the movement of marchers.

On Wednesday, medics aligned with the protest movement said security forces shot and killed a child in Bahri during neighborhood protests that have been taking place daily.

Thursday's six deaths — four in Omdurman, one in Khartoum and another child in Bahri — brought the number of protesters killed since the coup to 109. There were many injuries and attempts by security forces to storm hospitals in Khartoum where they were being treated, the Central Committee of Sudanese Doctors said.

There was no immediate comment from Sudanese authorities.

The United Nations envoy in Sudan, Volker Perthes, this week called on authorities to abide by a pledge to protect the right of peaceful assembly.

"Violence against protesters will not be tolerated," he said.

Military leaders said they dissolved the government in October because of political paralysis, though they have yet to appoint a prime minister. International financial support agreed upon with the transitional government was frozen after the coup, and an economic crisis has deepened.

Burhan said on Wednesday that the armed forces were looking forward to the day when an elected government could take over, but this could be done only through consensus or elections, not protests.

Mediation efforts led by the United Nations and the African Union have so far yielded little progress.