Four people have been arrested in Bahrain after an overnight attack on security forces that wounded four policemen, including an officer, authorities say.The unrest began on Friday when protesters blocked a highway linking a number of of predominantly Shia villages with the capital, Manama, a day after clashes on the second anniversary of the uprising against the kingdom’s Sunni rulersThe interior ministry said in a statement on Saturday that it had "arrested four terrorists possessing weapons with which they had fired on security forces in Karzakan”, a village southwest of Manama.A police officer and three policemen were taken to hospital after being wounded "by buckshot fired by a terrorist group”, Tarek al-Hassan, police chief,  said in statement.The security forces "had to respond to the attack to defend themselves”, Hassan added, without giving further details of the attack or the police response.The announcements came as thousands were expected to take to the streets of Manama for the funeral of a teenager the opposition said was killed in clashes between police and activists earlier this week.Bomb defusedIn another development, police said they found a bomb planted on a busy causeway linking Bahrain to Saudi Arabia.The 2kg bomb was discovered on Thursday near a mosque on the Bahraini end of the route, used by thousands of people a day, and was safely defused, the information ministry said on Saturday.Protests against the kingdom’s monarchy and government have stepped up since Thursday, and violence associated with the demonstrations has led to the deaths of the 16-year-old protester and a police officer.Bahrain has seen two years of political upheaval linked to opposition demands for a real constitutional monarchy, with the unrest claiming at least 80 lives, according to international rights groups.Agenices