STOCKHOLM (Reuters) - Lars Vilks, a Swedish blasphemer whose irreverent sketch of the Holy Prophet (PBUH) enraged many Muslims, was head-butted on Tuesday while giving a lecture about freedom of speech. Vilks said he was assaulted by a man sitting on the front row as he spoke at the University of Uppsala, about 70km from Stockholm. He head-butted me and I fell into the wall and lost my glasses, Vilks was quoted by news agency TT as saying. He added he was unharmed. A spokesman for Uppsala police said about 20 people tried to attack Vilks after interrupting his lecture, adding that the police had to intervene to stop them. Two people were detained. Blasphemer Vilks was not immediately available to comment further. In March an American who called herself JihadJane, was charged with plotting to kill the Swede and using the Internet to enlist co-conspirators.