The protest follows Tuesday's assassination of the governor of Punjab province, Salman Taseer, by one of his bodyguards, after the politician sought to reform the law that gives the death penalty for defamation of the Prophet Mohammad.
Pakistan's most high-profile political killing in three years has bitterly divided the country, horrifying moderates but winning praise from religious scholars and lawyers who festooned the presumed killer in garlands.
Two senior police officers in Karachi said more than 20,000 protesters had joined the rally and more were arriving, while senior police official Irshad Sehar told AFP that more than 30,000 people were taking part.
Banners at the event included some supporting Taseer's presumed killer, police commando Malik Mumtaz Hussain Qadri, who has been praised by religious conservatives for shooting his boss outside an Islamabad coffee shop.
"Mumtaz Qadri is not a murderer, he is a hero," said one banner in the national Urdu language.
"We are ready to