The fragile state of the sectarian divide in Iraqi politics was exposed today when a fight broke out in parliament after a mobile phone ringtone played a Shia Muslim chant.

A procedural session of the Iraqi parliament was suspended as Shia and Sunni leaders stormed out to protest the ringtone and the subsequent scuffle, which erupted between the armed bodyguards of the Sunni speaker Mahmoud al-Mashhadani and the hardline Shia politician, Gufran al-Saidi.

The mobile phone belonged to Ms al-Saidi, who is a member of the Islamist movement led by the radical Shia cleric Moqtada al-Sadr. According to Ms al-Saidi, one of her guards was holding her phone when it rang, playing a Shia prayer.

Mr al-Mashhadani sent one of his guards -- because of the risk of assassination in Baghdad, all Iraqi politicians come to parliament accompanied by armed men -- to ask her to turn it off. But the phone rang again, at which point a fight broke out in the lobby of the parliament building, with guards from both sides and a veiled Ms al-Saidi joining in.

Ms al-Saidi led a walkout on to the steps of the parliament building, where she told waiting television crews: "I demand an urgent investigation". She was joined by the independent MP, Mithal al-Alusi, a Sunni who leads the small Nation party, who said "those involved should be sued" and that bodyguards should be unarmed in parliament.

The incident will provide more fuel to Shia leaders who have already accused Mr al-Mashhadani, a Sunni who was appointed speaker last month to increase the representation of Sunnis in national politics, of being partisan and undiplomatic in his new role.

After 20 minutes, the protesting MPs were led back into the chamber by the outgoing Iraqi Prime Minister, Ibrahim al-Jaafari. Nouri al-Maliki, the incoming Prime Minister, said yesterday that he was on the verge of appointing a Cabinet, the last stage in the long and fraught process of creating a government of national unity.

Eighteen Iraqis were killed in scattered violence today. In the worst attack, gunmen opened fire on a bus full of employees travelling to a state-run electronics company in Baqouba, 55km (35 miles) northeast of Baghdad.

The gunmen stopped the bus, opened fire and threw a bomb on board, killing 11 people. The toll of yesterday's suicide attack in the northern city of Tal Afar rose today to 22 dead and 134 wounded, according to Iraqi authorities.

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