Egyptian authorities detained four activists on Thursday for putting up posters calling for nationwide protests on Friday, which the country\'s Islamists have vowed to stay away from. The four, including film director Aida al-Kashef, were held by military police Cairo as they put up posters calling for the Friday demonstration dubbed \"the second revolution.\" A military source said the four activists were released after several hours in detention. In a Facebook call, the Coalition of the Revolution Youth, which groups several movements behind the uprising that toppled president Hosni Mubarak in February, urged protesters to rally for \"an end to political corruption.\" Protesters want the acceleration of trials of former regime figures and their removal from top jobs in police, universities and other public institutions. They are also calling for a return of security forces to the streets, amid weeks of insecurity and sectarian clashes blamed on remnants of the old regime. But the Muslim Brotherhood, the country\'s largest and best organised opposition movement said it was \"very concerned\" by the call for protest. In a statement, the Brotherhood asked \"who are the people angry with now?\" The group said the revolution had achieved many of its goals, including not only the ouster of Mubarak, but also his referral to trial along with his sons and associates. The call to protest can therefore \"only mean that the anger is directed at the people themselves or at the army,\" said the Islamist group, urging protesters not to drive a wedge between the people and the military. For weeks, many activists have expressed frustration with the handling of the transition by the ruling Supreme Council of the Armed Forces (SCAF), which took power when Mubarak was ousted. The military said in a statement on Thursday that it will steer clear of protests in an effort to avert any unrest. It warned in a statement on its Facebook page of \"suspicious elements who will try to pit the military against the people,\" and said it \"decided to have completely no presence in areas of protests to avert these dangers.\" Activists say the military council has only agreed to put Mubarak and his sons on trial after intense street pressure, arguing that the momentum must be kept up for a transition to full democracy. On Monday, cyber activists launched \"No SCAF day,\" which saw nearly 400 blogs publish posts criticising the military council, which has also come under fire for alleged human rights abuses.