People have started to gather hours before the mass rally in the city's Yenikapi district, many carrying national flags.
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 all types of ideologies, regardless of their political preferences.
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). A collection of Turkish celebrities will also be at the event.
"Let the nation be there together," Erdogan said earlier on Sunday.
Suggested by Erdogan, 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 over 700 medical personnel will also be on duty.
Turkey's government has repeatedly said the defeated attempt was organized by followers of Fetullah Gulen, who has lived in self-imposed exile in the U.S. state of Pennsylvania since 1997.
Gulen is also 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.
