Under the changes to the electronic messages law, approved by parliament at first reading, Internet providers and mobile phone operators would be obliged to provide to police non-stop direct access to the electronic communication of all Bulgarians.
Police want fast access to the traffic data of crime suspects as the Balkan country's new center-right government, elected last July, struggles to tame powerful organized crime and rampant corruption.
Failing to show results may lead to more European Union sanctions on aid. Brussels cut Bulgaria's access to millions in aid in 2008 over its failure to punish corrupt officials and crime bosses.
But over 300 protestors, who rallied in front of parliament in Sofia shouting Bulgaria is not Big Brother, said the proposed legal changes violated human rights.
Seven million people do not deserve to fear they are being followed because of a drug dealer, a serial killer or whoever it might be, law student Simeon Simeonov, 22, said.
Interior Minister Tsvetan Tsvetanov said last week that the government would most likely make changes to the proposed legislation to address the criticism before parliament discussed the law at a second, final reading later this month.
(Reporting by Irina Ivanova; editing by Ralph Boulton)