US Senators have voted to begin debate on repealing and replacing Obamacare, a key campaign promise of President Donald Trump.

Demonstrators interrupted the start of the make-or-break vote with shouts of "Kill that bill, don't kill us" and "Shame" as they stood in the visitors' gallery.

They were led out of the chamber but could still be heard.

Senators voted to formally open debate on legislation to repeal much of the Affordable Care Act, a key achievement of President Trump's predecessor, Barack Obama.

The final tally was 51-50, with Vice-President Mike Pence breaking the tie.

Mr Trump hailed the vote as a "big step" and said Obamacare "should have been terminated a long time ago".

One of those who voted was Arizona Senator John McCain, who returned to Congress for the first time since being diagnosed with brain cancer and was given a standing ovation.

Mr Trump, who made getting rid of the legislation a key plank of his successful White House bid, had called on Republicans to "step up to the plate" on the issue after "seven years of talking".

Senate Majority Leader Mitch McConnell urged colleagues in his party, which controls both chambers of Congress, to vote in favour of moving forward on the legislation.

Democratic leader Chuck Schumer urged senators to reject it, saying: "Turn back now before it's too late."

The development sets up days of debate and votes, and it is unclear whether ultimately there will be enough votes to pass whatever bill is presented to Congress.

In a sign of the difficulties the Trump administration will face getting a bill passed, Senator McCain said after the vote that he would not vote for the legislation as it stands.

Last month, a non-partisan analysis found there could be 22 million more people without health insurance by 2026 in the US under the new healthcare proposals.

The estimate comes after the Congressional Budget Office (CBO) analysed Republican senators' draft 'Better Care Reconciliation Act'.

The previous House version of the bill to replace Obamacare would have left 23 million more Americans uninsured.