According to local media the armed assailant burst into the office and opened fire.
Turkey’s main opposition Republican People’s Party (CHP) runs the prosperous Sisli district.
It is not clear whether the shooting of the Jewish politician is linked to Friday’s military coup in which more than 200 people were killed.
Around 1,400 were wounded as soldiers commandeered tanks, attack helicopters and fighter jets in their bid to seize power, strafing parliament and the intelligence headquarters and trying to seize the main airport and bridges in Istanbul.
Forces loyal to Turkey’s President Recep Tayyip Erdogan quashed the coup attempt in a night of explosions, air battles and gunfire.
Turkish security forces are still searching for some of the soldiers involved in the coup bid in various cities and rural areas but there is no risk of a renewed bid to seize power, a senior security official told Reuters.
The official said Turkey’s military command had been dealt “a heavy blow in terms of organisation” but was still functioning in coordination with the intelligence agency, police and the government.
Some high-ranking military officials involved in the plot have fled abroad.
Erdogan has long accused Gulen of trying to create a “parallel state” within the courts, police, armed forces and media. Gulen, in turn, has said the coup attempt may have been staged, casting it as an excuse for Erdogan to forge ahead with his purge of the cleric’s supporters from state institutions.
Today the United States and European Union on Monday sternly warned Turkey to respect the rule of law after President Recep Tayyip Erdogan’s government launched a massive crackdown following the failed coup.
US Secretary of State John Kerry and EU foreign policy chief Federica Mogherini said in Brussels that Friday’s attempted putsch was “no excuse” for excessive action, as Turkish authorities said they had arrested over 7,500 people and sacked more than 9,000.