Participants are going through security checks before entering the rally area. Only Turkish national flags have been permitted, not those of political parties.
Smaller rallies will take place in every Turkish province. The public gatherings are expected to attract people from across the political spectrum.
The Yenikapi rally is to be addressed by President Recep Tayyip Erdogan and Prime Minister Binali Yildirim, who is also the leader of the ruling Justice and Development (AK) Party.
Leader of the main opposition Republican People’s Party (CHP) Kemal Kilicdaroglu and Nationalist Movement Party (MHP) leader Devlet Bahceli have confirmed they will attend the rally, which will start at 5 p.m. local time (1400GMT).
Bahceli is currently in the city to attend the event. A collection of Turkish celebrities will also be there such as world motorcycling champion Kenan Sofuoglu. He left Sakarya in western Turkey with a group of bikers.
“Let the nation be there together,” Erdogan said earlier on Sunday.
The rally was set up under the aegis of the presidency and organized by Istanbul Metropolitan Municipality together with the Office of the Governor.
The Yenikapi event will mark the end of weeks of “democracy watch” rallies held in cities across the country since the defeated coup, which martyred 240 people and wounded nearly 2,200 more.
A huge stage has been erected in the area, as well as television screens and sound systems.
Air-supported security measures have been taken around the rally point. Around 13,000 people, aside from police officers, will be on duty to organize the event and perform other technical duties.
Helicopters, ambulances and more than 700 medical personnel will also be on duty.
Turkey`s government has repeatedly said the coup was organized by followers of Fetullah Gulen, who has lived in self-imposed exile in the U.S. state of Pennsylvania since 1999.
Gulen is accused of leading a long-running campaign to overthrow the state through the infiltration of Turkish institutions, particularly the military, police and judiciary, forming what is commonly known as the parallel state.
Erdogan repeatedly said that he saw no difference between terrorist groups such as the PKK, PYD, YPG, and Daesh and U.S.-based preacher Fetullah Gulen`s FETO.