AN explosion caused by a homemade bomb rocked an Istanbul metro station overnight, triggering panic in the evening rush hour and wounding five people.

The local mayor and the state-run Anatolia news agency confirmed a bomb about 5.15pm Tuesday (2.15am Wednesday AEDT) caused the blast, which came at a time of growing jitters over security in Istanbul and the risk of a militant attack in Turkey’s largest city.

The blast, which was heard in several areas of the city, hit an overpass close to the metro station in the Bayrampasa district of Istanbul’s European side.

“Five of our citizens were injured when a pipe bomb left on barriers on the overpass exploded,” Atilla Aydiner, the district mayor, told A-Haber television.

Security sources were also quoted by the Dogan news agency as saying the blast appeared to have been caused by a homemade bomb.

The metro system in Istanbul was brought entirely to a halt after the blast, the municipality said, although normal service was gradually restored.

Earlier reports had said the cause of the blast could have been linked to an electrical transformer and that six people had been injured.

There was no immediate claim of responsibility.

“The cause of the explosion is not clear. We are investigating all possibilities,” Istanbul governor Vasip Sahin was quoted as saying by Anatolia.

The private Dogan news agency said the explosion occurred seconds after a bus carrying police drove past, suggesting that police could have been the target of the attack.

Aydiner said a number of vehicles were damaged in the blast, including a bus and a car. Authorities deployed riot police with shields to secure the perimeter of the site in case of a second blast.

Closed-circuit TV footage shows an explosion resembling a fireworks display in the dark, with sparks landing on the busy overpass (Watch below).

The agency’s footage also showed a white van on the overpass with large holes in its windshield.

VIDEO: CCTV footage of the explosion at metro station in #Istanbul tonight - @jmsardo pic.twitter.com/skMi0gr55a — Conflict News (@Conflicts) December 1, 2015

Sahin said the explosion caused panic because it occurred on a busy section of the road.

“We are thankful that there was no loss of life,” Sahin said.

Subway services at Bayrampasa and elsewhere were briefly disrupted following the explosion.

Witness Onur Dugenci said he was close to the metro when he heard “a very, very loud bang”.

“Later, my family and friends called from other parts of the city and said they could hear it, too,” Mr Dugenci told the BBC.

“There was no fire. But the explosion was so powerful that the windows on some cars were broken.”

The Anadolu Agency identified one of the injured as a 36-year-old man, but said he wasn’t in a serious condition.

Dogan news agency reported that many ambulances were heard rushing to the residential and industrial neighbourhood.

A municipal official told Reuters that train operations had been suspended after a loud noise was heard.

Photographs on social media showed dozens of people walking alongside overground train tracks after the trains stopped.

Although Istanbul’s metro is largely underground, it runs above ground in some places, including around Bayrampasa.

Turkey is currently on alert for attacks after 103 people were killed on October 10 when two suicide bombers ripped through a crowd of peace activists in the capital Ankara, the worst attack in modern Turkey’s history.

Metro/trains were stopped. Passangers on the railways now. No word on nature/cause of the explosion. pic.twitter.com/eWVNfFRM1I — taylieli (@taylieli) December 1, 2015

Kurdish rebels fighting Turkey for autonomy in the mainly Kurdish southeast of the country have carried out bombings in Istanbul in the past. A banned left-wing group is also active in Istanbul.

The banned ultra-left wing group the Revolutionary People’s Liberation Party-Front (DHKP-C) has also staged a string of usually small-scale attacks in Istanbul over the last months.

Tens evacuate & ran out of Bayrampaşa metro station after series of explosions a while ago.. #Istanbul #Turkey pic.twitter.com/5M90HLoE9f — Rami (@RamiAlLolah) December 1, 2015

Fighting between rebels belonging to the Kurdistan Workers’ Party, or PKK, and government forces flared up again in July, killing hundreds of people and derailing a fragile peace process with the Kurds.

URGENT: Istanbul trains suspended at one metro station after loud noise; local media reports explosion pic.twitter.com/Ne84h2SvxF — RT (@RT_com) December 1, 2015

There have been repeated security alerts on Istanbul’s expanding metro system in the last months but until now all had proved to be false alarm.

Also on Tuesday, hundreds of people were evacuated from areas around London Bridge station after a hoax caller reported a “suspicious item”.