The arrival of Pakistan's prime minister at the Supreme Court on contempt charges signals the start of a showdown between the court and the nation's civilian leadership.
Yusuf Raza Gilani's pleaded not guilty to the charge.
It comes from his refusal to re-open corruption cases against President Asif Ali Zardari, the head of Gilani's political party.
The civilian-judicial confrontation centres on the thousands of old corruption cases thrown out in 2007 under a new amnesty law passed under President Pervez Musharraf.
Zardari was the main beneficiary but the law was overturned by the court two years later.
Gilani and his advisors have refused to re-open the cases despite a court order to do so.
If convicted Gilani could be forced to step down and faces up to six months in jail.
The Supreme Court's action is being widely hailed by analysts and media in Pakistan where corruption tops the list of opinion polls as the nation's biggest problem.
Paul Chapman, Reuters
