There was a fragile calm in central Athens last night after several hours of extensive rioting that followed protests to mark the one-year anniversary of the shooting of a teenager by a policeman and as several hundred protesters remained holed up in the grounds of the University of Athens and the city's Law School. According to authorities, some 300 people remained on the campuses of the two institutions following the clashes with police. Officers were unable to enter the university campus due to the asylum law. Earlier, rioters, who used the two institutions as their bases, clashed from roughly 6 p.m. and for about three hours with thousands of police who had been deployed in the city center to ensure that the anniversary of the killing of Alexis Grigoropoulos by a special guard passed as peacefully as possible. However, following a calm protest by thousands of people which began at about 2 p.m., scenes reminiscent of those that followed Grigoropoulos's death last year began to play out on the capital's streets as rioters threw rocks, burned dumpsters and damaged a dozen stores. Police reacted by firing tear gas and detaining almost 150 people. Four citizens and 16 police officers were injured during the clashes. In one of the worst incidents, two policemen were dragged off their motorcycles and beaten over the head with metal chains. They were taken to the hospital with non-life threatening injuries. The rector of the University of Athens, Christos Kittas, was also admitted to the hospital after rioters stormed into his office. Kittas was struck on the head and was taken to the Ippocrateio Hospital, where doctors placed him in intensive care, saying that he had suffered a mild heart attack. The gates of Athens University had been locked to prevent the premises being used by rioters. A prosecutor yesterday launched an investigation into how they managed to force their way in. Police made a preemptive raid on an anarchist hangout in Keratsini near Piraeus on Saturday night. They discovered some 200 empty bottles they believe were going to be used to make Molotov cocktails, two jerry cans of fuel, two hammers, three sledgehammers, three helmets, 13 gas masks and a stun grenade. They also seized a computer. Officers arrested 22 people, two of whom were identified as the son and daughter of PASOK MP and deputy parliamentary speaker Grigoris Niotis. «I have total faith in Greek justice, just as I trust my children,» said Niotis in a statement.