In a post to almost 136,000 people, Imam Muhammed Asim Hussain said Mumtaz Qadri killed Salman Taseer "in honour of the Prophet".
Qadri was sentenced to death for the murder of Salman Taseer in 2011, a politician who strongly opposed the country's blasphemy laws, often used to target Christians.
He shot Mr Taseer in broad daylight while acting as a personal body guard. He didn't show any remorse for his actions.
Qadri was hanged on Monday for the crime.
It was "a dark day in the history of Pakistan," claimed Mr Hussain. Qadri had been "wrongfully executed and martyred in the way of Allah", he added.
Mr Hussain is a prominent cleric in Bolton and often speaks at mosques.
Tens of thousands of people attended Qadri's funeral on Tuesday whilst more protested across Pakistan.