The Supreme Court heard arguments Tuesday in two cases testing the police's authority to search detainees' mobile phones without warrants, in what could have a major impact on US law enforcement.

The closely watched cases at the top US court are a dramatic example of justice systems adapting to increasingly pervasive new technologies, particularly with smartphones able to contain reams of important personal information -- including photos and videos.

The nine justices on the prestigious panel will have to decide how to apply the 18th century US Constitution's Fourth Amendment protection against "unreasonable searches and seizures" to today's highly advanced data troves contained in mobile phones.

The US government argues that phones, like other personal items such as wallets or purses, may be searched during an arrest without a warrant, but privacy advocates are showing resistance.

About 12 million people are arrested every year in the United States -- many of them for trivial offenses -- while 90 percent of the population owns a mobile phone.

"People carry their entire life on cellphones," said Justice Elena Kagan.

"The police can seize the phone and look at the personal data because that person was arrested for driving without a seat belt?"

Justice Samuel Alito expressed the need "to balance the interests of privacy versus the law enforcement," particularly in light of the rapid advance of technology.

The hearing focused on two cases.

The first emerged from the arrest of David Riley, a Californian student pulled over in 2009. His car registration was found to have expired.

Police discovered loaded guns in his car and, after scouring his smartphone, entries related to a local gang and an earlier shooting.

His sentence of 15 years to life was upheld on appeal.

In the other case, Boston police in 2007 searched the call log of Brima Wurie's flip phone, which led them to an apartment where they found drugs, money and weapons.

An appeals court threw out the evidence found on Wurie's phone, saying it had been obtained unconstitutionally.

- 'Great deal of evidence' -

Michael Dreeben, representing the government, said mobile phones can potentially "contain a great deal of evidence" and argued that police should have the right to search phones without a warrant to avoid evidence on the phone being destroyed.

Challenging the government's position, Justice Antonin Scalia exclaimed: "It seems absurd to search in a cellphone for an arrest for not wearing a seat belt!"

The New York Times is among several news organizations that filed a brief supporting Riley and Wurie, saying mobile phone searches threatened news-gathering.

"Mobile phones aren't weapons and pose no physical threat," the Times said in an editorial on the eve of the Supreme Court arguments.

"For better or worse, mobile phones have become repositories of our daily lives, and will become only more powerful over time. As a rule, the police should have to get a warrant to search them."

Norman Reimer, executive director of the National Association of Criminal Defense Lawyers, said "allowing the police to search the contents of a cellphone without obtaining a warrant would be an affront to America's long history of individual privacy rights."

"It is akin to granting the police the right to search the most intimately personal details of our lives," he said ahead of Tuesday's arguments.

The Supreme Court has already acknowledged the need to adapt to new technologies, ruling in 2012 that authorities would violate the Fourth Amendment if they attached a GPS tracking device to a private car.

A final decision on the cellphone cases is expected by late June.