Pakistani Prime Minister Yusuf Raza Gilani has been formally charged with contempt at the country's Supreme Court.
He is accused of failing to reopen old corruption cases against President Asif Ali Zardari.
The prime minister, who faces jail and being disqualified from office if convicted, pleaded not guilty at the session in Islamabad.
He argues that the charges are politically motivated and that Zardari has immunity as head of state.
The president, who is accused of using Swiss bank accounts to launder bribes, denies the corruption allegations.
The accusations stem from Swiss cases dating back to the 1990s when Zardari's wife, the assassinated Benazir Bhutto, was prime minister.
In its rulings, the Supreme Court gave the attorney general until February 16 to submit evidence against Gilani. Those documents will be examined by the court next week. Gilani's defense will have until around the end of February to file documents in the case.
The case comes with Pakistan preparing to hold Senate elections on March 2, in which the ruling Pakistan People's Party (PPP) of Gilani and Zardari is seeking to gain a majority.
Outside the court, a senior member of the PPP, Qamar Zaman Kaira, criticized the indictment.
"The prime minister is the prime minister, and God willing he will remain so," Kaira said. "But certainly, today, we stand here with heavy hearts. We are in a very disturbing situation. For the first time, the prime minister of Pakistan has been charged. It is a sad day in the history of Pakistan."
A 2007 amnesty law passed under former President Pervez Musharraf protected Zardari and thousands of other politicians and officials from being prosecuted for corruption. But the Supreme Court overturned the amnesty law in 2009.
Reading out the charge sheet, Judge Nasir ul-Mulk said the prime minister had "willfully flouted, disregarded, and disobeyed" a Supreme Court order to write to authorities in Switzerland asking them to reopen the cases against the president.
Gilani drove himself and his lawyer to the court in an apparent attempt to show humility.
Security was tight, with helicopters buzzing through the sky, while hundreds of police blocked roads leading to the court building.
The case, seen as part of a long-running standoff between the government and the judiciary, has reportedly distracted the government from dealing with a host of problems, including a sluggish economy and an Islamist insurgency.
There are concerns that the case could result in a further boost in the influence of Pakistan's powerful military.
Compiled from agency reports