Hours after a 6.2 magnitude earthquake shook Turkey's biggest city, sending panicked residents racing onto the streets, Istanbul remained on edge late Wednesday as a string of aftershocks continued to rattle nerves. 

As night fell and temperatures dropped, some could still be seen camped out in parks and open spaces, while hundreds of students went to seek shelter on the campus of Istanbul Technical University.

"An earthquake of 6.2 magnitude occurred in Silivri, Sea of Marmara, Istanbul," Interior Minister Ali Yerlikaya said on X, adding that it was felt in the surrounding provinces.

The quake was followed by more than 120 aftershocks, some very powerful, the minister said, although there were no reports of major damage or serious injury. 

The initial quake struck at 12:49 pm (0949 GMT) at a depth of 6.92 kilometres under the sea, which lies to the south of the city, and lasted 13 seconds, he said. 

"By 5:55 pm (1455 GMT), 127 aftershocks had occurred, the largest of which was 5.9 magnitude," he said. 

As buildings shook, hundreds of people rushed onto the streets where many could be seen checking their mobiles for information or making calls. Others rushed to find safety in the city's handful of parks or other open spaces, AFP correspondents said.

"I just felt an earthquake, I've got to get out," a shaken-looking decorator, who did not want to give his name, said while fleeing a four-storey apartment where he was working near the city's Galata Tower. 

"We all panicked and just ran. There's absolutely nothing else we can do," a street seller called Yusuf told AFP. 

CCTV footage collated by Turkey's DHA news agency showed the moment when the quake hit, with staff and customers seen bolting for the door in fast food outlets, restaurants and a barber shop, some falling over in their haste to get out. 

Istanbul governor Davut Gul said nobody had died in the quake or the aftershocks but confirmed that hospitals were treating "151 people injured when they jumped or tried to jump from a height in panic".

The injuries were not life-threatening, he added.

An abandoned three-storey building in Fatih district collapsed without causing any injuries, he said. 

Schools and universities, which were closed on Wednesday as Turkey marked National Sovereignty Day, would remain closed until the weekend, the education ministry said. 

President Recep Tayyip Erdogan said he was "following the developments closely".

Tremors felt in Greece, Bulgaria capital

The tremors could be felt in all of the provinces surrounding the Sea of Marmara, in northeastern Greece and three Greek islands off Turkey's western coastline, and as far away as the Bulgarian capital of Sofia, according to officials and AFP journalists.

The quake hit 24 kilometres south of Silivri, a district on the megacity's western outskirts, which made headlines last month as the location where Istanbul's mayor Ekrem Imamoglu was jailed over a graft probe his critics say is politically motivated. 

A number of students are also being held there, arrested for joining the mass protests that erupted over the move against Imamoglu, Erdogan's biggest political rival. None of them was hurt, the Parents Solidarity Network said on X.

"The earthquake in Istanbul was most strongly felt in Silivri but our children are fine. There is no problem at the prison," the group wrote.

Some of Istanbul's southern districts lie just 15 kilometres (nine miles) from the North Anatolian Fault.

Many Istanbul residents are still haunted by memories of the last "Big One" that struck part of the city in 1999. 

Around 20,000 people were killed in two massive quakes that devastated Turkey's densely populated northwest -- including parts of Istanbul -- three months apart in 1999 as the eastern strand of the fault line ruptured.

Seismologists have calculated a 47-percent chance of an earthquake with a magnitude above 7.3 hitting Istanbul within 30 years.

The last tremor to be felt in Istanbul was in mid November, when a quake caused brief panic but no damage or injuries.