Khaama Press reported the protesters call Valentine's Day an anti-Islamic holiday that encourages unmarried men and women to live together.
The protesters said the country should celebrate a "day of modesty" instead.
Pakistan's main religious party, Jamaat-e-Islami, organized the demonstration.
"The law enforcing agencies must prevent such gatherings, otherwise we will stop in our own way," Shahzad Ahmed, the head of the local organization, said.
Khaama Press said Valentine's Day is becoming increasingly popular with younger Pakistanis, who are taking part by giving chocolates, cards and flowers to their sweethearts.