The laws, one of which makes rape victims liable to prosecution for adultery unless they produce four male witnesses, were introduced by a military dictator in 1979 and have since drawn widespread criticism from rights activists.
The ruling party introduced a bill in parliament this week aimed at amending the laws, to the fury of religious conservatives.
Mairaj-ul-Huda Siddiqui, a leader of an opposition alliance of six religious parties, told a crowd of about 200 in the city of Karachi after Friday prayers: "We will even sacrifice our lives for this and will not allow these amendments to take place,"
"This is part of a US and Jewish conspiracy and we will resist it forcefully," Siddiqui said.
Similar small protests were held in Lahore, Peshawar and the capital, Islamabad.
Mian Aslam, a member of parliament for the religious party alliance, told a crowd of about 50 in Islamabad that President Pervez Musharraf was bowing to pressure from the United States and Britain.
Force would be used to oust the government if the changes went ahead, he said.
While conservatives oppose any amendment to the laws, rights activists have long been demanding their complete repeal.
Musharraf, who promotes an ideology of "enlightened moderation", has assured rights activists he will back moves to amend or repeal the laws.
But with an election due by the end of next year, critics fear the government could lose its resolve should it need any favours from the conservative opposition.
"We will even sacrifice our lives for this and will not allow these amendments to take place"
Mairaj-ul-Huda Siddiqui, a leader of an opposition alliance of six religious parties
The laws, enforced in 1979 by the then military ruler Mohammad Zia-ul-Haq, laid down punishments for crimes such as rape, theft and adultery.
Under the code, a man and woman found guilty of having sex outside marriage could be sentenced to death by stoning or 100 lashes, while thieves would have their right hand amputated.
But those punishments have seldom been invoked, let alone carried out.
The amendment bill has been sent to a parliamentary committee for scrutiny. It is not clear when it will come up for debate.