At least 24 people, including children, drowned and a dozen were missing on Monday when an overloaded boat taking migrants towards EU waters sank in the Black Sea just off Istanbul, Turkish officials said.

Those on board were mainly Afghans in search of a better life in the EU. They had reportedly paid several thousand euros each to people smugglers for a seat on the overloaded vessel.

Thousands of people on Saturday took to the streets in Turkey in an international day of solidarity with the mainly Kurdish Syrian town of Kobane which has been besieged by jihadists for over six weeks.

Around 1,000 pro-Kurdish supporters took part in a march in central Istanbul which was so far peaceful following warnings from the authorities they would not tolerate any unrest, an AFP correspondent said.

Suspicious envelopes containing a yellow powder were sent Friday to the Belgian, Canadian and German consulates in Istanbul, prompting a security alert, officials said.

One person who opened the envelope at the Canadian mission was directly exposed to the unknown substance, which was now being analysed by experts, Turkey's disaster management agency AFAD said.

The jailed leader of Turkey's outlawed Kurdistan Workers' Party (PKK) said he saw a better chance for a peace deal with Ankara, despite Kurdish protests over Turkish inaction in Syria.

"My hopes over a successful execution in this process have been increased," Abdullah Ocalan said in a message relayed on Tuesday by pro-Kurdish lawmakers who visited him in his prison on the island of Imrali near Istanbul.

"It was as if we were in the middle of a war zone," said 22-year-old drama student Aslihan Celebi as she described the scene of a violent anti-jihadist protest at Istanbul University.

The university has in the last few weeks been the center of violent clashes between leftist students denouncing the brutality of Islamic State (IS) jihadists and the group's sympathizers.

Turkish Prime Minister Ahmet Davutoglu has reportedly labeled Istanbul's Taksim Square, which was rocked by deadly anti-government protests last year, "as the ugliest square in the world".

"If anyone tells me that Taksim is a perfect square, then I would tell him that it is the ugliest square in the world," Davutoglu told reporters.

Turkey warned on Saturday it would not hesitate to strike back at Islamic State jihadists if they attacked Turkish troops stationed at Ankara's exclave inside Syria.

"We will absolutely not hesitate to respond if something happens there," President Recep Tayyip Erdogan told reporters in Istanbul, referring to the tomb of Suleyman Shah, the grandfather of the founder of the Ottoman dynasty Osman I.

Turkish police on Wednesday detained three students at Istanbul University suspected of being sympathizers of the Islamic State (IS) group following clashes at an anti-jihadist campus protest, reports said.

Police stormed the university's faculty of humanities and science in Istanbul's Beyazit district after Islamist students armed with sticks and stones attacked another group protesting against IS.

Prosecutors in Istanbul charged four construction workers with manslaughter on Thursday over a dramatic workplace accident that saw a lift in an unfinished luxury skyscraper plummet 32 storeys, killing 10.

Turkish riot police on Sunday fired tear gas and water cannon at protesters in Istanbul, a day after 10 workers were killed when a lift crashed to the ground from the 32nd storey of a building.