Baghdad, Aug 30 (UNI) At least 15 people have died in clashes between Iraqi security forces and supporters of a powerful Shia cleric in Baghdad overnight. Officials say hundreds have been injured after protesters loyal to Muqtada al-Sadr stormed the presidential palace. The Monday violence was sparked after Sadr announced his retirement from politics. Iraq's caretaker prime minister has called for calm and the military has declared a nationwide curfew after unrest in several other cities, the BBC said. Street fighting erupted as gunfires were exchanged and tracer rounds illuminated the night sky in what is being called the worst violence to hit the Iraqi capital in recent years. Much of the fighting has been concentrated around the city's Green Zone, an area that houses government buildings and foreign embassies, the BBC added. Security officials said some of the violence was between the Peace Brigades, a militia loyal to Sadr, and members of the Iraqi military. Videos shared on social media appeared to show some fighters using heavy weaponry, including rocket-propelled grenades (RPGs). Iran has closed its borers with Iraq, and Kuwait has urged its citizens to leave the country immediately. BBC quoted an international agency reporting that medics claimed 15 supporters of Sadr were shot dead and about 350 other protesters injured. A spokesperson for UN Secretary General Antonio Guterres said he was alarmed by events and called for "immediate steps to de-escalate the situation". Interim Prime Minister Mustafa al-Kadhimi, an ally of Sadr, has suspended cabinet meetings and has pleaded with the influential cleric to intervene and stop the violence. A senior Sadr aide told Iraq's state news agency INA that he had announced a hunger strike until the violence and use of weapons stopped. The violence sparked a day after Sadr's announcement that he was withdrawing from political life -- a move he blamed on the refusal of rival Shia leaders and parties to reform the Iraqi political system. In October, candidates loyal to Sadr won the most seats in Iraq's parliament, but he failed to secure enough seats to form a government. He has since refused to negotiate with Iranian-backed Shia groups, sparking almost a year of political instability. Sadr said in a statement: "I had decided not to interfere in political affairs, but I now announce my final retirement and the closure of all (Sadrist) institutions." Some religious sites linked to his movement will remain open. Sadr, 48, has been a dominant figure in Iraqi public and political life for the past two decades. His Mehdi Army emerged as one of the most powerful militias which fought US and allied Iraqi government forces in the aftermath of the invasion which toppled former ruler Saddam Hussein. He later rebranded it as the Peace Brigades, and it remains one of the biggest militias which now form part of the Iraqi armed forces.