Waving Iraqi flags and chanting "Death to America! Death to Israel", the demonstrators mounted convoys of buses and headed for the capital on Thursday, some of them wearing white shrouds symbolising their readiness to accept martyrdom.
The demonstration is to be held following Friday prayers in the teeming Baghdad suburb of Sadr City, home to some two million people, and comes at a tense time for the capital.
Sahib al-Ameri, general director of the Foundation for God's Martyrs, part of Sadr's movement said: "Thousands of Najaf people have set off for Baghdad as a response to Moqtada al-Sadr's invitation to take part."
Anger over Israeli attacks on Lebanon - which target Hezbollah but have caused widespread civilian casualties - has raised passions in Iraq, where Shia militias have an uneasy relationship with the US-backed government.
US forces have visibly tightened security around Sadr City, a working class Shia district where Sadr's supporters are openly armed.
"Fifteen-hundred of Sadr's followers from Basra have arrived to take part in the demonstration that had been called in Baghdad," said Sadr spokesman Akil al-Bahadli.
Blast kills 9
Meanwhile, a roadside blast has killed at least nine people in al-Amin, an eastern district of Baghdad, a police source says.
The powerful blast, which also left 14 wounded, hit Shorjah market in an area selling electrical appliances, sending a dirty plume of yellow dust up over the city skyline.
Violence in Baghdad is claiming
100 lives a day
The attack near Rusafi Square in the shopping area of Rashid Street apparently targeted vendors and commercial stalls, said police Lt Ahmed Mohammed Ali.
He said the bomb was hidden in a parked motorcycle.
Another police source said casualties appeared to be civilians, rather than members of the security forces.
The attack occurred as Taro Aso, the Japanese foreign minister, was visiting Baghdad.
Violence in Iraq's capital and other restive areas is claiming around 100 lives every day.
Thursday's attack followed a bomb blast in Baghdad on Wednesday evening next to a soccer pitch which killed 12 and wounded 13, most of whom were players and spectators.