According to the new law, telecom companies must record and store for six months information such as dates and numbers of phone calls. In March, the addresses of e-mails will also be stored under the same law. Authorities can request the stored information after a person is indicted for an alleged crime with court approval.
Authorities said the contents of conversations and e-mails will not be recorded and the data will help in the investigation of serious crimes and terrorism. But opponents fear that it might be abused.
By Wednesday, more than 5,000 Romanians had signed an online petition demanding the law be scrapped.
Director of democracy advocacy group Transparency International Victor Alistar said the law could lead to a reduction in freedoms. Daily Romania Libera called it the "Big Brother" law.
"Prosecutors and intelligence services will use the data ... according to their own rules and regulations," the paper said.
Senate leader Mircea Geoana acknowledged the law could cause "fear or suspicion that it could be used as a tool to consolidate authoritarian tendencies in Romania."
"We still have in the back of our minds the memory of a state that used to oppress people and crush their basic freedoms," he said.
The law states that it complies with European Union measures to fight terrorism and serious crimes. The Parliament voted for it in November.