The rally was organised by a coalition of NGOs, trade unions and business groups, who warned that demonstrators displaying or voicing political emblems or slogans would be excluded.
It comes three days before a more politically charged anti-terrorism rally in Istanbul, to be addressed by President Recep Tayyip Erdogan.
Erdogan's two-month-old offensive against the militant Kurdistan Workers' Party (PKK), with whom he had been in peace talks for three years, has divided Turkey.
The PKK has hit back hard, killing scores of soldiers and police in a string of attacks in the mainly Kurdish southeast, triggering reprisal attacks by nationalist mobs on Kurdish parties and businesses in other regions.
The organisers of Thursday's rally, who included the Tusiad business federation, TESK trade union and the Turkish Bar Association, called for national unity.
"This rally is seeking to demonstrate unity and brotherhood of 78 million people under the Turkish flag," TESK chairman Bendevi Palandoken told AFP.