A suicide attack has killed at least 53 Shia Muslim pilgrims and left another 137 wounded in Iraq's southern city of Basra, local police say.

The attacker, wearing a suicide vest, was said to have been disguised as a policeman when he targeted pilgrims who were passing through a checkpoint on the outskirts of the city on Saturday.

The pilgrims were heading towards a Shia mosque in the busy al-Zubair district, west of Basra city. The mosque is home to a seventh century shrine.

The attacks on Shia Muslim worshippers have been blamed on al-Qaeda linked forces.

Al Jazeera's Jane Arraf, reporting from Baghdad, said Iraqi security forces have "been on high alert for attacks of this kind" surrounding the Arbaeen rituals, commemorating the death of Imam al-Hussayn.

The attack is the lastest in a wave of car bombings and suicide attacks targeting Shia pilgrims on their way to Karbala for Arbaeen rituals, a religious rite that occurs 40 days after the day of Ashura.

The Arbaeen pilgrimage, one of the largest in the world, has drawn 12 million congregants to Karbala itself.

With the pilgrimage becoming a major focus of attacks, our correspondent said, some pilgrims "wrap themselves in white shrouds" - the traditional burial attire for Muslims - along the way.

Iraqi security forces have stationed 30,000 police and soldiers outside Karbala city and along the roads leading to it.

Iraq has experienced political tension along ethnic and sectarian lines after the Shia-dominated cabinet of Nouri al-Maliki issued an arrest warrant last month for Vice President Tariq al-Hashemi, the country's highest-ranking Sunni politician.

Hashemi, who is currently living in Iraq's Kurdish north, was charged with terrorism as the final US troops were leaving the nation.

Basra was also the site a protest last week to denounce a decision by President Jalal Talabani, a Kurd, to shelter Hashemi.