In a nondescript, creeper-draped building in the capital of Islamabad, a small team of men is purging Pakistan's Internet.
Shadowy government officials are blocking thousands of pages deemed undesirable. But they are not fast enough. So the government is now testing Canadian software that can block millions of sites a second.
The censorship helps shape the views of 180 million Pakistanis on militancy, democracy and religion. Online debates dissect attacks by U.S. drone aircraft, the uneasy alliance with the United States and prospects for peace with arch rival India.
But activists say liberal voices are increasingly silenced while militants speak freely. They worry customised filters will only deepen that divide.
"Secular, progressive and liberal voices are being increasingly targeted," said Shahzad Ahmad, the founder of Bytes For All, which campaigns for internet freedoms from a small, crowded office. "Anything can be banned without debate."
An internet provider who declined to be identified said the number of banned pages doubled in five years, partly a reaction to cartoons or films offensive to Muslims.
Citizen Lab, a research centre at the University of Toronto, published a report in June showing the Pakistani government was testing filtering software supplied by Canadian firm Netsweeper.
The Pakistani government and Netsweeper declined to comment.
In 2012, the government circulated a five-page document seeking filtering software.
"Pakistani ISPs and backbone providers have expressed their inability to block millions of undesirable websites using current manual blocking systems," the government said in the paper, a copy of which was seen by Reuters. It said it needed a system "able to handle a block list of up to 50 million URLs".
Activists from Bolo Bhi, an internet freedom group whose name means "Speak Up", said Pakistan wanted the strict online censorship practiced in its ally China.
About 42 million Pakistanis can get online, the government says, and the Internet is one of the few places where they can speak freely, said Bolo Bhi director Farieha Aziz. Twitter helps voters reach leaders directly.
"Now Pakistanis can get direct access to politicians. Previously they were just on television, telling you stuff," Aziz said.
Bolo Bhi asked technology companies to refuse the bid and said U.S.-based