Pakistani PM Yousuf Raza Gilani has been charged with contempt in an appearance before the country's Supreme Court.
Mr Gilani is accused of failing to re-open corruption cases against President Asif Ali Zardari.
The prime minister, who faces jail and being barred from office if convicted, pleaded not guilty.
Mr Gilani argues that the president, who denies the corruption charges, has immunity as head of state.
President Zardari is accused of using Swiss bank accounts to launder bribes.
The Supreme Court has said Mr Gilani defied a court order to write to the Swiss authorities and ask them to re-open the cases against Mr Zardari.
Mr Gilani is expected to argue that he received legal advice that it would have been unconstitutional to pursue the cases.
Mr Zardari says the charges against him are politically motivated.
Helicopters hovered overhead and hundreds of riot police were guarding the Supreme Court as Mr Gilani arrived.
The case has now been adjourned until 28 February. Mr Gilani will not have to appear on that date.
This could now become a lengthy process as even if Mr Gilani is convicted, he will have the right to appeal, reports the BBC's Aleem Maqbool in Islamabad.
The case is part of a stand-off between the government and the judiciary, which many believe is being backed by the military as it pursues the case against the civilian administration.
Many are concerned that the case will distract Mr Gilani's focus from Pakistan's many complex problems to his own survival, our correspondent adds.
"It's a sad day for Pakistan," Qamar Zaman Kaira, a senior member of Mr Zardari and Mr Gilani's Pakistan People's Party (PPP), said outside court, according to AFP.
In 2009 the Supreme Court overturned an amnesty dating from the period of former President Pervez Musharraf which protected President Zardari and hundreds of other politicians from being prosecuted for corruption.
In an interview with Al-Jazeera television last week, Mr Gilani acknowledged he would have to stand down if convicted.
"If I'm convicted, then I'm not even supposed to be a member of the parliament," he said.