President Zardari denied any breach of the constitution
Pakistan is facing further political uncertainty after the Supreme Court blocked an attempt by the president to appoint two new judges.
President Asif Ali Zardari issued an order appointing a Supreme Court justice and a High Court judge.
But the Supreme Court struck the order down, saying it appeared to violate the constitution as the president had not consulted the court beforehand.
Mr Zardari's spokesman denied that he had breached the constitution.
Farhatullah Babar said the chief justice had been consulted, as required by the constitution.
The Supreme Court will hold a hearing on 18 February.
This latest confrontation has added to the tension and sense of instability, reports BBC correspondent Orla Guerin in Islamabad.
In December, the Supreme Court threw out an amnesty that protected Mr Zardari from allegations of corruption dating back to the 1980s.
On Sunday, Prime Minister Yousaf Reza Gilani said there was no link between the appointments and the amnesty ruling.
"I want to give a message to the nation that the country's institutions are strong, and we will work within our domains," he told reporters, the Associated Press news agency said.
"There is no threat to democracy. There is no threat to the country nor to any institution," he added.
The controversial law granting senior politicians amnesty was brought in by ex-President Pervez Musharraf.
The court's decision in December opened the way to possible prosecution for Mr Zardari's political allies, although he is still protected by presidential immunity.
Pakistan's Western allies will be concerned about fresh turmoil at a time when they want the country to focus on fighting the Taliban and al-Qaeda, our correspondent adds.