An explosion - suspected to have been caused by a suicide bomber - has hit the popular Istiklal Street in central Istanbul's Taksim square area, local media report, with several deaths reported.
The district governor said at least four people were killed and 20 were injured in Saturday morning's explosion, apparently including the suspected suicide bomber.
Footage from the scene showed police and emergency services cordoning off the street, which has been completely cleared of people.
Istiklal. street is a long pedestrian thoroughfare that winds its way through the Beyoglu neighbourhood from Taksim Square.
It is rimmed by hundreds of shops and would have been filled with pedestrians at the time of the explosion.
Al Jazeera's Jamal Elshayyal said the location of the explosion was the equivalent of a bomb going off in Oxford St in London or Fifth Avenue in New York.
The explosion comes as Turkey is on edge following two recent suicide bomb attacks in the capital, Ankara, which were claimed by a Kurdish group, that is an off-shoot of the outlawed Kurdistan Workers' Party, or PKK.
Turkey has been fighting on multiple fronts. As part of a US-led coalition, it is battling the Islamic State of Iraq and the Levant (ISIL, also known as ISIS), which has seized territory in neighbouring Syria and Iraq.
It is also battling the PKK in its southeast, where a two-and-a-half-year ceasefire collapsed last July, prompting the worst violence since the 1990s.
Turkey sees the unrest in its largely Kurdish southeast as deeply linked to events in northern Syria, where the Kurdish YPG militia has seized territory as it fights both ISIL and rebels battling President Bashar al-Assad.
ISIL has carried out at least four bomb attacks on Turkey since June 2015, including a suicide bombing which killed 10 German tourists in central Istanbul in January.