* Sparks protests by those who see killer as defender of the faith
* Blasphemy laws carry death sentence for insults to Islam
* More than 100 charged with blasphemy in Pakistan each year
Protests broke out within hours by supporters of the killer, who consider him a hero who defended the faith,
Mumtaz Qadri, a bodyguard of Salman Taseer, governor of Punjab province, shot him dead in the capital, Islamabad, in 2011.
"Qadri was hanged at around 4:30 a.m.," senior police officer Rizwan Omar Gondal told Reuters by telephone. The execution took place at the Adiala jail in the city of Rawalpindi outside Islamabad.
Taseer had championed the cause of a Christian woman sentenced to death in a blasphemy case that arose out of a personal dispute. Taseer had said the law was being misused and should be reformed.
Late in 2011, an anti-terrorism court handed down a double death sentence to Qadri for murder and terrorism. The sentence was appealed and upheld by the Supreme Court late last year.
More than 100 people are charged with blasphemy each year in predominantly Muslim Pakistan, many of them Christians and other minorities.
Conviction of blasphemy carries a death sentence, although no one has yet been hanged for the charge.
Controversy over the law has exposed the growing gap between religious conservatives and liberals in Pakistan, with hard-line religious leaders considering Taseer a blasphemer himself for even criticising the law.
Some lawyers showered Qadri with rose petals when he first arrived in court days after the killing. The judge who first convicted him was forced to flee the country after death threats.