Secretariat sources said Aloke Saha, an employee in the receiving section of the CMO, had to be rushed to the hospital.
The police have detained the attacker, identified as Animesh Mishra, a resident of Krishnagar, for interrogation.
The sources said that the man was denied entry as he did not have any appointment papers.
The accused told the police that he had been a visitor to the Writers' Buildings since 2006 and wanted to meet the Chief Minister "to make a complaint that his land had been grabbed during the previous Left Front government."
The man said all he wanted was to meet the Chief Minister "to get justice."