The legal challenge to Zardari over his two posts does not pose an immediate threat to the unpopular president but it is a reminder of the legal difficulties he faces, legal analysts said.
The Pakistan Lawyers Forum (PLF) filed a petition, or a challenge, questioning the right of the president to hold the two offices and in response, the High Court in the city of Lahore ordered Zardari's principal secretary to explain.
"Since the president could not appear because of security reasons, the court asked his principal secretary to appear in court on May 25," PLF president A.K. Dogar told reporters outside the court.
There is no constitutional bar on the president holding office in a political party but Dogar said the Supreme Court had in the past barred a president from holding a party post.
"Our Supreme Court judges decided in 1993 that the president should be non-partisan. He should not involve himself in political battles. He should shun politics but here he is a party head, which is illegal," he said.
Zardari, the widower of assassinated former prime minister Benazir Bhutto, is co-chairman of her Pakistan People's Party (PPP), which emerged the biggest party after a February 2008 general election and heads a ruling coalition.
Their son, Bilawal Bhutto Zardari, who is a student in Britain, is the other party co-chairman.
The president, dogged by corruption accusations stemming from the 1990s, when Benazir Bhutto served two terms as prime minister, has struggled to win the popularity his wife enjoyed.
His political enemies question his legitimacy to rule and some want to see old corruption cases against him revived, even though he enjoys presidential immunity.
Nuclear-armed Pakistan is a key ally of United States as it struggles to bring stability to neighbouring Afghanistan.
Political turmoil, which a sustained challenge to Zardari would likely incite, would distract the government from its campaign against Islamist militants on the Afghan border and dismay the United States and other allies with troops in Afghanistan, and worry investors.
Zardari handed over most of the president's powers to the prime minister last month, partly to mollify his critics.
Nevertheless, he retains considerable political influence as head of the PPP and vulnerable to various legal challenges.
The Supreme Court, which in December threw out a controversial law that had protected Zardari and others, including Interior Minister Rehman Malik, has called for old corruption cases against him to be revived.
The court has taken up a case against the government for not seeking the revival of money-laundering cases against Zardari in Swiss courts.
Zardari spent 11 years in jail on various charges but was never convicted.
