Pakistan's Supreme Court has begun hearing a challenge to a law introducing a Taleban-style moral code in North-West Frontier Province (NWFP).
Hardline politicians say they have popular backing for the law
President Pervez Musharraf has asked the Supreme Court to declare the new law unconstitutional and a breach of people's fundamental rights.
The law includes measures to ensure people respect calls to prayer and to discourage singing and dancing.
The NWFP government says it was mandated to pass the law when elected.
But the Pakistan government says the Hisba (Accountability) law would project a negative image of the country abroad, particularly in the West.
"It's likely to create a parallel judicial system and altogether a separate and parallel administrative system," federal law minister, Wasi Zafar, told the BBC.
The BBC's Roland Buerk in Islamabad says the timing of the case comes at a bad time for President Musharraf, who is cracking down on extremism and trying to reassure the world that Pakistan is moderate.
"Islam call for brotherhood, unity, well being and learning," the state-run Associated Press of Pakistan quoted President Musharraf as saying.
"Please do not believe those who support extremism. They want to push Pakistan backward."
But religious parties say they are simply following the will of the people.
"This is our right to reform society and its ills according to the ideology and according to the traditions and customs and teachings of the Koran," Qazi Hussain Ahmed, leader of the Jamaat-i-Islami, told the BBC.
The law is aimed at moral policing
"And we have consulted everybody so there should be nothing against the constitution."
The Hisba law was passed by the NWFP assembly in July, with 68 votes in favour and 34 against.
Under the new law, an Islamic watchdog will monitor the observance of Islamic values in public places in NWFP.
'No singing and dancing'
The plan is reminiscent of the infamous Department of Vice and Virtue, set up by the Taleban regime in Afghanistan.
Under the new law, the principal duty of the cleric, called "mohtasib" - one who holds other accountable - will be to ensure people respect the call to prayers, pray on time and do not engage in commerce at the time of Friday prayers.
He will also stop unrelated men and women from appearing in public places together, and discourage singing and dancing.
One of his tasks will be to monitor the media to ensure "publications are useful for the promotion of Islamic values".
The Department of Vice and Virtue set up by Afghanistan's former ruling Taleban became the focus of criticism from human rights organisations.