A series of ceremonies began early in day to culminate in a state funeral.
Mr Ecevit, a political force in Turkey for half a century, died last Sunday at the age of 81 after almost six months in a coma following a stroke.
Mr Ecevit was probably best known for ordering Turkish troops into Cyprus in 1974 after a Greek-backed coup.
His funeral was moved from Wednesday to allow more people to attend and they began lining the streets from early on Saturday.
"People's Ecevit! Turkey is proud of you," chanted the mourners.
The crowd, some weeping, showered the hearse bearing his coffin with flowers, and many waved Turkish flags, banners and portraits of Mr Ecevit, a political force in the country for half a century.
Around 10,000 police officers are on duty and have closed roads for the funeral procession.
The square outside Ankara's biggest mosque was crammed full of people from all over Turkey, many with his photograph pinned to their chests.
Many Turks say they will remember their former prime minister as a modest and honest man from the political left-wing.
Following a change in the law made this week, Mr Ecevit will be buried at Ankara state cemetery, alongside the founders of the republic.
There has been some speculation the funeral of this staunch secularist could develop into a protest against the government, which has its roots in political Islam, the BBC's Sarah Rainsford reports from Ankara.
Mr Ecevit was prime minister when the EU accepted Turkey as an accession candidate and when the Kurdish separatist leader Abdullah Ocalan was caught.
But his last years in office were beset by his own ill-health and an ailing economy.
His party suffered a landslide defeat in 2002, allowing the current government to sweep to power.
