The student wing of the Jamaat-e-Islami (JI) rallied outside the press club in the northwestern city of Peshawar, chanting slogans against Valentine's Day, saying it had "spread immodesty in the world" .
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Valentine's Day is increasingly popular among younger Pakistanis, many of whom have taken up the custom of giving cards, chocolates and gifts to their sweethearts to celebrate the occasion.
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But Pakistan remains a deeply traditional society where many disapprove of Valentine's Day as a Western import.
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The demonstrators, who carried a banner urging February 14 to be a "day of modesty", said the tradition encouraged unmarried men and women to live together in sin
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"We will not allow holding of any Valentine's Day function," Shahzad Ahmed, the local head of the student organisation, said.
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"The law enforcing agencies must prevent such gatherings, otherwise we will stop in our own way."