A CNN report said thousands took part in the demonstration Sunday in the southern port city of Karachi, with some demonstrators carrying banners in support of Muhammad Mumtaz Qadri, the personal security guard accused in last Tuesday's assassination of Punjab Gov. Salman Taseer, who wanted to change the law.
The rally, which led to the closure of Karachi's main roads and shops, warned the government against changing the law, the U.S. news network said.
Under the blasphemy law, in force for about three decades, any insult against Islam or the religion's founder, Prophet Mohammed, is a crime. Punishment can include the death penalty.
Police have quoted Qadri as saying he killed Taseer since "he did blasphemy of the Prophet Mohammed."
The rally was held even as civil rights activists lodged a complaint against a Karachi-based cleric for hate mongering and inciting violence, The Hindu newspaper reported.
The demonstration in support of the blasphemy law followed reports the government could consider amending it, The Hindu said.
In other developments, India's PTI news agency reported a Pakistani police officer, a member of the security detail of Pakistan's prime minister, was detained in Taseer's assassination. An official of Pakistan's electronic media watchdog also was detained.
The PTI report said authorities also were looking for a cleric who allegedly motivated Qadri, who surrendered after the shooting.