Tension between the Muslim Brotherhood and opposition was ratcheted up following renewed attacks on protesters camping outside Al-Ittihadiya presidential palace on Saturday. The attackers hurled firebombs at the protesters' tents and fired shots in clashes that left 23 people injured, including seven policemen. Following the clashes the Interior Ministry announced that police had arrested four men. In a Facebook statement the ministry said that investigations had revealed the attacks occurred following a scuffle between one of the detainees and a protester. The defendant returned to the scene of the scuffle accompanied by neighbours and acquaintances who threw Molotov cocktails and fired birdshots at the protesters. Four tents set up by protesters near the palace were torched. “The defendants appear to have no political affiliation,” the ministry of Interior said in a statement. “The attack seems to be criminally rather than politically motivated.” Karim Al-Shaer, one of the protesters, said that around tens of “thugs” had assaulted demonstrators and fired rounds in the air to terrorise them. “The security forces watched the perpetrators while they were attacking us. They only intervened when a soldier was shot,” he said. Violence extended to surrounding streets before the situation was contained. Central Security Forces were later deployed in the area to prevent any further violence. The Health Ministry said seven policemen, including one officer, and 16 civilians were injured in the clashes. Some sustained eye injuries caused by shotgun pellets. Injured protesters were transferred to nearby hospitals. Opposition protesters have been camping outside the presidential palace in Heliopolis since 4 December to protest against the Constituent Assembly and the draft constitution. Popular Current member Amr Al-Soudi accused the Muslim Brotherhood of being behind the attack. “We do not believe the assailants had no political motives. Everybody knows the sit-in is against President Morsi. The only people who would attack the protesters are either supporters of the president or thugs who had been paid to do so.” Activist Hossam Moenas believes the Muslim Brotherhood is increasingly adopting the same tactics employed by the Mubarak regime against its critics. “They do not send their members to attack the protesters but pay thugs to do the job instead,” he says. “It is clear that there were orders to the police not to intervene until the thugs finished their attack.” The Muslim Brotherhood, Moenas argues, wants to end the sit-ins in front of the presidential palace and in Tahrir Square before 25 January, the anniversary of the revolution. The 6 April Movement, meanwhile, blames security forces and not the Brotherhood for the attacks. “Security forces failed to protect the protesters and allowed the thugs to shoot at them,” said the movement's spokesperson Engi Hamdi. She added that the 6 April Movement, which is participating in the sit-in, had refused to accuse the Muslim Brotherhood of attacking the protesters. Hours after the clashes hundreds of Ultras joined the sit-in. The football fan group chanted anti-Muslim Brotherhood slogans. “We will sacrifice our lives to prevent attacks on peaceful protesters in Egypt,” the group chanted. Presidential adviser for political affairs Bassam Al-Zarqa denied any Brotherhood involvement in the attack, claiming that there are many groups seeking to hinder Egypt's democratic progress by showing that the presidency is unable to protect peaceful protesters. “These groups are targeting state institutions, especially the presidency and the ministry of interior,” he told Turkish news agency Anadolu on Sunday. Tension also grew as clashes between protesters and the public were reported in Tahrir Square which has been blocked to traffic for 55 days, causing major congestion in Downtown Cairo. On Saturday some members of the public tried to re-open the square by removing barricades which protesters had erected. Fights erupted between car drivers and protesters. Following talks with Interior Ministry officials protesters agreed to open the square to traffic between 6am and 10pm. They also announced that they will not end their sit-in until 25 January, when the square will host opposition protests. The opposition plans large demonstrations across the country to mark the anniversary of the revolution.