Armed clashes persisted on Sunday across Turkey's southeast, where an operation by Turkish forces intensified on the sixth day of a campaign that security sources said had resulted in the death of 110 Kurdish militants.

Protests erupted in Istanbul and Diyarbakir, the biggest city in the country's southeast, with hundreds demonstrating against the military operations. Police fired tear gas and rubber bullets to disperse the crowds.

Most of the fighting took place in Cizre and Silopi, towns near the Iraqi and Syrian borders that have been under curfew for almost a week. Nusaybin and Dargecit in the border province of Mardin and the historical Sur district of Diyarbakir have also seen fierce battles.

More Cizre footage. Black smoke rising over the town after fighting as security forces impose curfew. — John Beck (@JM_Beck)December 20, 2015

Although rooted in the countryside, militants of the Kurdistan Workers' Party (PKK) have shifted focus in recent years to towns and cities in the southeast, digging trenches and setting up barricades in streets to keep security forces away.

Security sources and residents said around 300 houses in Cizre had been damaged by the clashes and undetonated mortar shells lay inside buildings.

Electricity was cut in many neighborhoods in Silopi as power transformers were damaged. Food and drinking water were running scarce, residents said.

A two-year ceasefire between Turkey and the PKK fell apart in July, shattering peace talks and reviving a conflict that has ravaged the mainly Kurdish southeast for three decades.

"We will not get tired," Prime Minister Ahmet Davutoglu said in a speech to his ruling party's supporters. "We will fight day and night until all mountains, cities, districts and neighborhoods of this country are cleansed of terror centers."

The latest security operations, which media reports said were being conducted with 10,000 police and troops backed by tanks, were the largest since the end of the ceasefire.

Tanks deployed on hills surrounding Cizre have shelled PKK targets inside the city, while a military convoy of 30 armored vehicles stormed one of the districts.

Hundreds of people in Istanbul as well as Diyarbakir and eastern city of Van took to the streets to protest against the security operations and curfews.

Police fired tear gas, water cannons, and rubber bullets to disperse a crowd chanting "Long Live Kurdistan" in Istanbul's Taksim square, Reuters witnesses said. Several people across three cities were detained.

One Turkish soldier hurt in clashes in Cizre on Saturday has died of his wounds, security sources said. A postman working for the state mail company has also died after PKK militants attacked his vehicle on the highway to Sirnak, they said.

The PKK, which launched its insurgency in 1984, is designated as a terrorist group by Turkey, the United States, and the European Union.