Pakistan's top court on Wednesday acquitted a Christian woman who was sentenced to death on blasphemy charges in 2010, a landmark ruling that sparked protests by hard-line Islamists and raised fears of violence.

Chief Justice Mian Saqib Nisar announced the verdict to a packed courtroom and ordered Asia Bibi released. She has been held at an undisclosed location for security reasons and is expected to leave the country.

The charges against Bibi date back to a hot day in 2009 when she went to get water for her and her fellow farmworkers. Two Muslim women refused to drink from a container used by a Christian. A few days later, a mob accused her of blasphemy. She was tried, convicted and sentenced to death.

The mere rumor of blasphemy can ignite mob violence and lynchings in Pakistan, and combatting alleged blasphemy has become a central rallying cry for hard-line Islamists.

Salman Taseer, the governor of Punjab province, was shot and killed by one of his guards in 2011 for defending Bibi and criticizing the misuse of the blasphemy law. The assassin, Mumtaz Qadri, has been celebrated as a martyr by hard-liners since he was hanged for the killing, with millions visiting a shrine set up for him near Islamabad.

Ahead of the verdict, Khadim Hussain Rizvi, a hard-line cleric who has brought tens of thousands of people into the streets for past rallies, called on his supporters to gather in all major cities to express their love for the prophet and to protest if Bibi is released. Authorities have stepped up security at churches around the country.

Shortly after the ruling, hundreds of Islamists blocked a key road linking the city of Rawalpindi with the capital, Islamabad. Islamists gathered in Pakistan's largest city, Karachi, in the northwestern city of Peshawar and elsewhere. Police urged demonstrators to disperse peacefully.

In the eastern city of Multan, police arrested several demonstrators after clashes.

Paramilitary troops deployed in Islamabad to prevent protesters from reaching the Supreme Court, where security for the judges was being beefed up.

Bibi's family and her lawyer say she never insulted the prophet. In previous hearings her attorney, Saiful Malook, pointed to contradictions in testimony from witnesses. The two Muslim women who pressed charges against Bibi denied they quarreled with her, saying her outbursts against Islam were unprovoked.

Chaudhry Ghulam Mustafa, a lawyer for one of the plaintiffs, rejected the verdict, saying Bibi had confessed to making derogatory remarks against the prophet to seek pardon.

The three-judge panel upheld the blasphemy law itself, saying it was consistent with verses from Islam's holy book, the Quran. But they said prosecutors had failed to prove that Bibi violated the law. In addition to citing the Quran, the judges also referenced Shakespeare's King Lear, saying Bibi was 'more sinned against than sinning.'

Critics of the blasphemy law have said it is used to settle personal scores or to attack minority communities. Bibi's case was closely followed internationally amid concern for Pakistan's religious minorities, who have frequently come under attack by extremists in recent years.

Bibi's husband hailed Wednesday's verdict.

'I am very happy. My children are very happy. We are grateful to God. We are grateful to the judges for giving us justice. We knew that she is innocent,' said Ashiq Masih.

'My wife spent so many years in jail and we hope that we will soon be together in a peaceful place,' he said.