Iraq has been hit by weeks of political turmoil surrounding Prime Minister Haider al-Abadi's efforts to change the government.
Both Washington and the United Nations have warned the crisis could distract from the fight against the Islamic State (IS) group, which carries out frequent bombings against civilians.
The car bomb, which also wounded at least 38 people, struck a road in the Nahrawan area used by Shiite pilgrims who are walking to the shrine of Imam Musa Kadhim in northern Baghdad for annual commemorations, officials said.
IS considers Iraq's majority Shiites to be heretics.
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Kadhim, the seventh of 12 imams revered in Shiite Islam, died in 799 AD. The pilgrimage has in recent years turned into a huge event that brings the Iraqi capital to a standstill for days.
IS overran large areas north and west of Baghdad in 2014, but Iraqi forces backed by US-led military assistance have since regained significant ground.
The jihadists still control a large part of western Iraq, and are able to carry out frequent attacks against both civilians and security forces in government-held areas.
And four more were burned or shot to death when mobs torched houses and a Sunni religious endowment building after rumours of a suicide bomber sparked panic among a crowd of pilgrims.
The bombing came as hundreds of people turned out in Baghdad for a demonstration aimed at pressuring the government to carry out reforms, the latest in a series of such protests in the capital.
Demonstrators gathered at Baghdad's Tahrir Square and near the heavily-fortified Green Zone, where the government is headquartered.
"We are sending a message to parliament that today is the last chance" to accept "complete governmental change," said Hassanain Ali, one of the demonstrators.