Southern protesters torched offices of Sudan's ruling party after Khartoum police arrested three southern leaders and dozens of protesters on Monday in a crackdown against a pro-reform demonstration. Pagan Amum, Yassir Arman and Abbas Gumma from the former rebel Sudan People's Liberation Movement (SPLM) were freed after a few hours and were celebrating their release at the movement's headquarters in Khartoum along with thousands of supporters. But earlier angry protesters had reacted to their arrest by torching the offices of the National Congress Party (NCP) of President Omar al-Beshir in the southern cities of Wau and Rumbek, two provincial capitals, a southern government official said. There were no reports of casualties. Amum is the SPLMs secretary general, Arman its deputy secretary general in northern Sudan, and Gumma is a state minister at the country's interior ministry. Arman sustained minor injuries while in custody. At a press conference after their release, he said he was beaten by police. 'They took me to the toilet and they began to beat me until my clothes were torn,' Arman said at the party headquarters. Amum for his part vowed to continue protesting 'until the (peace agreements) are fully implemented...and peace, justice and freedom are achieved in Sudan.' Southern president Salva Kiir condemned the arrests, saying they broke the Comprehensive Peace Agreement (CPA) of 2005 that ended a devastating 22-year civil war between north and south. 'These arrests are not only provocative but unjustified, because the interim national constitution of the Sudan and the CPA allow for peaceful and democratic procession,' said Kiir, who is also first vice president in a national unity government in Khartoum. 'We call upon all Sudanese people to remain calm... and exercise their constitutional rights of expression within the law,' he said. Kiir said he had spoken to the Sudanese president, who had promised the release of all detainees. Police had announced that the demonstration to push for reforms ahead of national elections next year and an independence referendum for south Sudan scheduled for 2011 would be considered illegal. Police clashed with the several hundred protesters in Khartoum and its twin city Omdurman, using tear-gas and beating them with batons before the crowds dispersed, witnesses said. Among those arrested were Siddig al-Turabi, son of veteran Islamist opposition leader Hassan al-Turabi, and Khartoum state minister for health Barmina Awrial, along with more than 70 of the demonstrators. Four daughters and two grandchildren of another opposition leader, former prime minister Sadiq al-Mahdi, were also arrested, his son Mohammed Ahmed said. Calm had returned to the capital's streets by the afternoon. Khartoum police issued a statement published on the Sudanese Media Centre website, which is close to the security services, denying they used tear-gas or force to put down the protest. Security forces blocked roads leading to parliament, with a heavy presence in key areas. They also closed the bridge across the Nile linking Khartoum and Omdurman. The SPLM and Beshir's NCP have failed to agree on democratic reforms ahead of next April's elections and on a procedural law for the south's referendum scheduled for January 2011. The national vote will be the first in Sudan since the 1986 election which brought Mahdi to power. Beshir toppled his government in a coup three years later. The SPLM and around 20 opposition groups called for a 'peaceful protest' to put pressure on the NCP. Registration for the presidential, parliamentary and state elections began on November 1 and was extended until Monday at the request of opposition parties, including the southern former rebels. Khartoum state announced the closure of schools on Monday and a day off for public employees to underline the government's 'engagement ... towards democratic reform' and to aid voter registration. Reform and changes to the electoral law were key aspects of the 2005 peace accord which ended Africa's longest-running civil war. 