Lucknow: Over a dozen prisoners were injured in violence at the Muzaffarnagar jail late Friday night, police said. Three prison inmates were critically injured and are undergoing treatment. While the situation was brought under control by morning, police said tension prevails and the security was enhanced within the jail. It all began, officials said, with a verbal spat beteen Gayyur and Shahrukh gang supporters, soon it was free for all and the groups attacked each other with shaving razors, sticks and rods. The groups assaulted prison guards who fled to escape the wrath of the rioting prisoners. Jail superintendent Rakesh Singh said, adding that the situation was under control. There were incidents of violence and rioting in at least four jails in last two months. During a jail riot in Varanasi prison officials were injured and held hostage. Following this, at five prisons, provincial armed constabulary was handed over the security of the jails.