CAIRO - Protesters and security forces clashed at the presidential palace on Friday as thousands of people rallied in cities across the country against Egypt’s divisive Islamist leader President Mohamad Mursi.The Republican Guard appealed for calm as demonstrators hurled petrol bombs over the palace walls and security forces responded with water cannon and tear gas and troops fired warning shots into the night sky.Egypt’s official MENA news agency quoted a Republican Guard statement as saying protesters had tried to enter the palace grounds. Police then advanced on the protesters in armoured vehicles, firing tear gas.President Mursi vowed security forces will “act with utmost decisiveness” to protect state buildings. The presidency, in a statement posted on his Facebook page, also said it would hold “politically accountable” opposition groups behind the violence.It said protesters had tried to break down the palace gates and scale its walls, and demanded that opposition groups denounce the violence and call on their followers to withdraw.People took to the streets in a show of opposition to Mursi and his Muslim Brotherhood after a wave of deadly unrest swept the country last week in the worst violence since Mursi was elected president last June.Protesters marched through rare rainfall towards the capital’s iconic Tahrir Square and the presidential palace, chanting “Freedom!” and “Mursi is illegitimate!”Scores of protesters skirmished with riot police several hundred metres (yards) from the square, witnesses said, and two were wounded by birdshot fired by police. The clashes were less intense than in last week’s unrest which resulted in 56 deaths, mostly in Port Said, where violence erupted after 21 residents were sentenced to death on Saturday over football-related violence a year ago.Thousands of people protested in the canal city on Friday calling for Mursi’s removal, unfazed by a strong military presence, and similar marches also took place in the Mediterranean port city of Alexandria.The opposition National Salvation Front (NSF) joined rival factions on Thursday in condemning violence and supporting efforts for a national dialogue. But the coalition of mainly liberal and leftist groups also called Friday’s protests demanding a unity government and amendment of the Islamist-drafted constitution which polarised the nation when it was passed in December.The opposition accuses Mursi of betraying the revolution that toppled president Hosni Mubarak two years ago and took him to the presidency. “We’re here because we are sick of people lying to us,” said protester Ahmed Metwalli in Tahrir Square. “I voted for Mursi. I thought the Brotherhood, more than any group, would have understood the concept of injustice and try to fix it, but they failed,” the 27-year-old engineering graduate added.In a statement, the NSF said Egyptians would demonstrate nationwide against “a regime that seeks to impose its will on the people and is managing the country in the interest of the Brotherhood.”Top Islamic scholar Grand Imam Ahmed al-Tayyeb chaired Thursday talks between liberal opposition heads, Islamists, youth groups, independents and church members at the headquarters of Al-Azhar, Sunni Islam’s highest seat of learning.Participants signed an Al-Azhar document supporting “a serious dialogue” and condemning violence, while also calling on police to “protect citizens.” The crisis has sapped the popularity of Mursi — who won the June election with a slender majority — and complicated negotiations for a crucial $4.8-billion IMF loan that could help bail out the teetering economy.A presidency statement welcomed Thursday’s Al-Azhar agreement as “an important step on the road to re-establishing stability.” The interior ministry said it respected “the right to peaceful protest,” but urged all political forces to ensure “a peaceful and civilised” day on Friday, a spokesman said before violence flared again in the capital.