KHARTOUM (Reuters) -Four protesters were 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 towards the presidential palace, witnesses said.

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

The protests mark the third anniversary of huge demonstrations during the uprising that overthrew long-time 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 that have called on the army to quit politics.

Some 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 Sudanese military’s economic holdings.

Earlier, protesters barricaded some of the capital’s main thoroughfares with stones and burning tyres.

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 hamper the protest movement.

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

BRIDGES SHUT

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.

In recent days there have been daily neighbourhood protests in the build-up to Thursday’s rallies.

On Wednesday, medics aligned with the protest movement said security forces shot dead a child during protests in Bahri. Thursday’s four deaths, all in Omdurman, brought the number of protesters killed since the coup to 107.

There was no immediate comment from Sudanese authorities.

The United Nations envoy in Sudan, Volker Perthes, called this week 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. As a result, however, international financial support agreed with the transitional government was frozen and an economic crisis has deepened.

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

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

(Reporting by El Tayeb Siddig and Khalid Abdelaziz; writing by Aidan Lewis; editing by Alex Richardson and Mark Heinrich)