WASHINGTON - The final public ceremony honouring late US senator John McCain began Saturday in Washington, where eulogies by two former presidents will highlight the warrior politician's call for healing, but current commander-in-chief Donald Trump is notably absent.

Republican George W. Bush and Democrat Barack Obama will deliver remarks honouring their friend and former White House challenger, at a memorial service in Washington's National Cathedral that McCain planned himself in recent months as he battled brain cancer.

McCain, who died last Saturday at age 81, has been lionized over the past week of extraordinary and emotional memorials and tributes, including his congressional colleagues bestowing him the rare honour of lying in state in the US Capitol on Friday.

They were set to continue at the nationally televised cathedral ceremony, a who's who of political Washington attended by the likes of dozens of congressional colleagues, Bill and Hillary Clinton, former vice presidents Al Gore and Dick Cheney and former secretaries of state Madeline Albright, John Kerry and Henry Kissinger, who is to address the gathering.

The president's daughter Ivanka Trump and her husband Jared Kushner were also present, along with Defense Secretary Jim Mattis, White House Chief of Staff John Kelly and Hollywood icon Warren Beatty.

Earlier Saturday McCain's flag-draped casket was taken by honour guard from the US Capitol and placed in a black hearse, which stopped at the Vietnam Veterans Memorial to allow his widow Cindy McCain to lay a wreath honouring all of those who died in the conflict.

At the National Cathedral, attention will no doubt focus on the addresses by Bush and Obama.

That the men who vanquished McCain in their presidential battles were asked to speak is testament to his commitment to looking beyond party and signaling that Americans, regardless of political affiliation, are rowing together in the same boat.

But the absence of Trump, whose bitter feud with McCain has wrangled America's already stormy politics, will serve as a final rebuke to the president, highlighting the clash between a Republican elder statesman and the current president from his own party.

Former US presidents Barack Obama and George W. Bush, who both defeated John McCain in their White House contests, said Saturday at the late senator's funeral that he made them "better."

Hailing his friend as "an extraordinary man," warrior and patriot who embodied what is best in America, Democrat Obama said McCain "made us better presidents - just as he made the Senate better, just as he made the country better." He was echoing similar sentiments expressed minutes earlier by Republican Bush, who defeated McCain in a "hard fought" Republican primary battle in 2000, only to see that bitter rivalry melt away into a lasting friendship. "He also made me better," Bush said.

Trump arrives at golf course as funeral underway

US President Donald Trump on Saturday went to one of his golf courses as the final public ceremony honoring late US senator John McCainwas under way in Washington.

The president's motorcade arrived at Trump National Golf Club in Loudoun County, Virginia around 11:16am (1516 GMT) while eulogies to McCain were being delivered at his funeral at Washington's National Cathedral.

Earlier, McCain's daughter Meghan had delivered a tearful address which pointedly took aim at the president's campaign slogan.

"The America of John McCain has no need to be made great again because America was always great," she said to applause from an audience that included Ivanka Trump and Jared Kushner, the president's daughter and son-in-law.

Trump meanwhile was seen wearing a "Make America Great Again" cap on Saturday morning.

McCain was one of Trump's sharpest critics, and made clear in one of his final wishes as he struggled with brain cancer that he did not want the president to attend his funeral.

Trump waited several days after McCain's death to praise him directly and belatedly lowered flags to half-staff across the country only after bowing to pressure.

The roots of their feud go back to when Trump announced his candidacy for the Republican presidential nomination in June 2015, suggesting that many Mexican immigrants were criminals and "rapists."

McCain denounced him for using language that "fired up the crazies," while Trump said McCain was a "dummy" who had barely managed to graduate from the US Naval Academy. He went on to attack McCain's service in the military, saying of the onetime prisoner of war: "I like people that weren't captured."