American liberals and libertarians rarely see eye to eye -- but they have united behind demands for more transparency following recent revelations of vast and secretive surveillance programs.

The unlikely alliance has brought together the Democratic Party's far left and the ultraconservative Republican Tea Party, both of which are suspicious of the programs, which US officials insist are needed to prevent terror attacks.

During hearings this week with the directors of the National Security Agency and the Federal Bureau of Investigation, both sides took to task their parties' congressional leaders, who had long known about the spying programs.

"The mere fact that some members may have been briefed in a classified setting does not indicate our approval or support of these programs," said John Conyers, one of the most liberal Democrats in the House of Representatives.

"It's not a partisan concern and it is one that applies both to the present administration and to the last one, as well," he said.

"It's my fear that we are on the verge of becoming a surveillance state, collecting billions of electronic records on law-abiding Americans every single day."

The hearings came after government contractor Edward Snowden leaked details of a program to mine telephone logs and another that acquired data from several Internet giants, including Apple, Google, Microsoft and Facebook.

A few hours after Conyers spoke, Republican Rand Paul, a Tea Party favorite and lifelong fan of the libertarian philosopher Ayn Rand, announced he was filing a lawsuit against the NSA for abuse of power.

"I want to catch terrorists as much as any American," Paul said, flanked by Tea Party lawmakers and an official from the American Civil Liberties Union.

"But what separates us from them is the rule of law."

-- A debate, not a revolution --

"This is a rather unusual alliance for sure," said Christopher Arterton, a political scientist at George Washington University.

But "there has always been a bit of harmony between the two ends of the political spectrum about big government," he told AFP.

Arterton recalled that another unlikely coalition rose up to oppose, in 1913, the creation of the US central bank and, toward the end of the 19th century, the institution of the gold standard.

But in today's Congress far-right and far-left lawmakers don't wield much power, and the parties' leadership have promised a debate on the surveillance programs -- not a revolution.

Moreover, public opinion on the issue is fairly mixed.

A majority of Americans disapprove of the systematic gathering of a huge trove of telephone metadata, according to two recent polls by Gallup and CBS.

But when asked in a way that evoked the fight against terrorism, the numbers flipped.

According to a Pew study, 56 percent viewed it "acceptable" for the National Security Agency to track telephone records of millions of people "to investigate terrorism."

"Americans generally disapprove, in theory, of the database collection programs, but a majority are willing to accept it under certain circumstances in order to fight terrorism," explained Frank Newport, chief editor at Gallup.

"We get differences based on wording, when Americans aren't sure and it's a new, complex issue," he said.

The spying measures currently being debated were approved by large bipartisan majorities in the wake of the September 11, 2001 terror attacks.

In 2008, then-senator Barack Obama voted in favor of the law that gave birth to the PRISM program, in which the NSA gathers Internet data for investigations that it insists are only aimed at non-American terror suspects living abroad.