Alliance of European Conservatives and Reformists summit in Antalya showed solidarity with Turkey against attacks.
An explosion believed to have been caused by a suicide bomber has hit the popular Istiklal Street in central Istanbul’s Taksim square area.
Government officials said at least five people were killed and 36 were injured – seven seriously – in Saturday morning’s explosion.
Israeli Prime Minister Benjamin Netanyahu confirmed that at least two Israeli nationals were killed in the blast.
Two American nationals have also been killed, while Iran said that one of its citizens had also died.
It was not immediately clear if the two Americans had dual citizenship or were additional deaths.
“The United States condemns in the strongest possible terms today’s terrorist attack in Istanbul, Turkey,” National Security Council spokesman Ned Price said in a statement.
The suspected suicide bomber is also believed to have been killed. Twenty-four foreign citizens were among the wounded, including 11 Israelis.
Prime Minister Ahmet Davutoglu said the blast was “inhumane” and would not stop Turkey, which has been targeted by Kurdish rebels and the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) in recent months, from fighting “centres of terrorism”.
Footage from the scene showed police and emergency services cordoning off the street, which has been completely cleared of people.
Witnesses told Al Jazeera that hundreds of people ran in panic away from the site of the explosion, moments after the incident.
There was no immediate claim of responsibility.
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 explosion was the equivalent of a bomb going off in Oxford Street in London or Fifth Avenue in New York.
The explosion came with Turkey on edge after two recent suicide bomb attacks in the capital, Ankara, which were claimed by a Kurdish group which 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 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.