A bipartisan group of US senators finally rolled out an immigration reform bill Thursday, insisting it was the best chance in a generation to fix a broken system, but admitting perils lie ahead.

The comprehensive reform effort, filed with the Senate this week, is a huge measure aimed at bringing 11 million undocumented workers out of the shadows and onto a pathway to citizenship, while securing the nation's southern border.

The eight senators -- four Democrats and four Republicans -- came together over the span of three months for 24 closed-door negotiation sessions to hammer out the most significant immigration legislation in a quarter century.

"Today is just the beginning of our voyage," Senate Democrat Chuck Schumer told a news conference delayed by Monday's Boston terror attacks.

"It will be long and arduous, there will be perils we can't even anticipate," he warned, flanked by other members of the "Gang of Eight" while more than two dozen stakeholders, including union bosses, big business representatives, farm workers and evangelicals, stood behind him on stage.

Senate Majority Leader Harry Reid is aiming to bring the sweeping bill to the Senate floor by June. At least three hearings, and hours of likely intense debate on the Senate floor are expected.

"We're either going to get a bill or one helluva fight," Republican Lindsey Graham quipped.

The clashing has already kicked off.

Republican Jeff Sessions has long voiced his opposition to the bill, which he derided on Thursday as a "presumption of amnesty" that has "no border requirement" -- two assessments that the "Gang of Eight" hotly disputes.

Sessions is opposed to providing a pathway to citizenship, claims the bill's guest-worker program is too generous at a time of high US unemployment, and winces at the excess benefits he believes the government would provide undocumented workers.

Under the bill, undocumented migrants who can prove they have been in the country since before December 31, 2011, would get a legal temporary status and would be allowed to work, travel and drive without fear of deportation.

After 10 years, these immigrants could file for a green card and become permanent residents. Three years after that, they could request citizenship.

But to convince hardline Republicans opposed to the idea of amnesty, the Senate negotiators included ambitious measures to tighten security along the 1,800-mile (3,000-kilometer) border with Mexico.

They want to avoid a repeat of 1986, when Republican president Ronald Reagan approved reforms that led to amnesty for 2.7 million people but, because of a lack of border resources, did little to stem the tide of illegal arrivals.

Republican Senator Marco Rubio, a Cuban-American and potential 2016 presidential candidate, has emerged as the face of the movement, and on Thursday went out of his way to urge citizens to come together on the immigration bill.

"It's tragic that a nation of immigrants remains divided on the issue of immigration," he told reporters.

"Let's bring these people out of the shadows. They'll undergo a background check, they'll pay a fine, they'll start paying taxes, they won't qualify for federal benefits," he added.

Senator John McCain agreed, but acknowledged the uphill climb toward final passage in the Senate and then the House.

"There's a long and difficult road ahead," he said.

The Senate recently rejected a controversial measure that would expand background checks for gun buyers, but Schumer insisted it did not bode ill for immigration legislation, which he said has far broader support.

"I think the majority in both caucuses really want to get this done," Schumer said. "I believe that this is ours to lose."