BAGHDAD, Iraq - Campaigning began Saturday for Iraqi parliamentary elections with some candidates embroiled in controversy after aides pulled down pictures of fighters killed in combat with jihadists and replaced them with their own posters.

Around 7,000 candidates have registered to stand in the May 12 poll, with 329 parliamentary seats up for grabs.

As the clock struck midnight on Friday, volunteer workers spread across the country to put up posters of the candidates, AFP reporters said.

"At midnight sharp, they began pulling down pictures of the martyrs and replaced them with posters of the thieves," said Baghdad resident Settar Tourki.

Martyrs refer to the thousands of government forces and allied militiamen killed fighting to push back the Islamic State group after the jihadists launched a sweeping offensive in 2014.

Pictures of these forces killed in combat dot the streets of Baghdad and major cities and towns.

Social media users cried foul as pictures of politicians, referred to by some in Iraq as "thieves" over allegations of corruption, replaced those of combatants.

Facebook user Laith al-Shommari denounced what he called an "insult" to the memory of fighters "who have sacrificed their lives so that we can live in peace and security".

"We should burn the electoral posters of all these cowards and opportunists," he said referring to the candidates.

The May elections will be the fourth parliamentary and provincial assembly polls since the ouster of dictator Saddam Hussein in a US-led invasion in 2003.

Iraq is the 12th most corrupt country in the world, according to international rankings.