Iraqi security forces killed four protesters in Baghdad on Friday and forcibly dispersed activists blocking the main port near Basra, as the country's top cleric called for electoral reforms to end the unrest.

Security forces opened fire and launched tear gas at protesters on a central Baghdad bridge, police sources said. Two people died from bullet wounds and two from tear gas canisters launched directly at their heads. At least 61 more were injured.

The prime minister's military spokesperson denied any protesters had been killed on Friday.

In the south, security forces reopened the entrance to Iraq's main port, Umm Qasr, which protesters had blocked since Monday, port sources said. Normal operations had not yet resumed.

At least 330 people have been killed since the start of mass unrest in Baghdad and southern Iraq in early October, the largest demonstrations since the fall of Saddam Hussein in 2003.

Protesters are demanding the overthrow of a political class seen as corrupt and serving foreign powers while many Iraqis languish in poverty without jobs, healthcare or education.

Iraq's top Shi'ite Muslim cleric, Grand Ayatollah Ali al-Sistani, called on Friday for politicians to hurry up in reforming electoral laws because the changes would be the only way to resolve weeks of deadly unrest.

"We affirm the importance of speeding up the passing of the electoral law and the electoral commission law because this represents the country moving past the big crisis," his representative said during a sermon in the holy city of Kerbala.

Reforms proposed by President Barham Salih would have lawmakers elected from individual districts instead of whole provinces, and stand as individuals rather than on party lists, to satisfy demands of protesters to reduce the power of parties and have greater local representation. But a bill proposed this week by Prime Minister Adel Abdul Mahdi omitted those changes.