President Joe Biden launched a fiery attack on his "dangerous" November presidential election rival Donald Trump in Thursday's State of the Union address, warning that US democracy is under "assault."

In the dramatic start to the speech, Biden said he wanted to "wake up the Congress and alert the American people" to the danger.

"Not since President Lincoln and the Civil War, have freedom and democracy been under assault at home as they are today," he said. "What makes our moment rare is that freedom and democracy are under attack at both at home and overseas."

Then homing in on Trump, he said the Republican was "bowing down" to Russian President Vladimir Putin, and he vowed to cheers from Democrats: "I will not bow down."

Letting the punches fly, Biden never said Trump's name, instead referring repeatedly to him as "my predecessor, a former Republican president."

In what was one of the most important speeches of his long political career, the 81-year-old Democrat hoped that his vigorous and at times humorous delivery will ease concerns about his age as he prepares to face Trump in eight months.

He arrived on the floor of the Capitol, crammed with congressional members, Supreme Court justices and government leaders, to extended cheers from supporters and chants of "four more years."

But in a sign of the multiple challenges facing Biden -- including from the Democratic left -- protesters against his support for Israel's war on Hamas tried to block the motorcade from the White House to Congress.

The annual set-piece presidential speech, which in past years has lasted from around an hour to more than 90 minutes, was a unique chance for Biden to pitch his reelection message in front of his closest political allies and foes -- and a national TV audience.

Some of the Republicans booed and heckled Biden but he shot back with jokes and mocking what he said were their plans to cut taxes for billionaires.

Trump, 77, has a narrow lead over Biden in opinion polls but he faces multiple criminal charges linked to his efforts to overturn his 2020 election loss and refusal to give back boxes of top secret documents after grudgingly leaving the White House.

- 'Greatest comeback' -

In his speech, Biden slammed Republican abortion opponents, saying they have "no clue about the power of women in America," in what Democrats see as a key vote-winning issue.

And he touted the booming US economy, even if Americans are still unhappy with high prices and polls show many saying their economic fortunes have not improved.

Biden said that "in thousands of cities and towns the American people are writing the greatest comeback story never told."

"I inherited an economy that was on the brink," he said. "Now our economy is literally the envy of the world. Fifteen million new jobs in just three years -- a record. Unemployment at 50-year lows."

The Democrat was also expected to take on the war in Gaza and fury among leftists and the Arab-American community over his strong support for Israel's war against Hamas.

He was to announce during the speech that he has ordered the US military to set up a port on Gaza's coast to bring in more aid, reflecting the acute political pressure from many in his own party.

Ahead of the speech, Trump accused Biden of "horrific devastation" during his three years so far in office, focusing on the record numbers of migrants crossing the Mexican border.

"It's time to tell Crooked Joe Biden -- you're fired," Trump said in a video "prebuttal", using the catchphrase of his former TV reality show "The Apprentice."

- Age concerns -

For the millions of Americans tuning in to the spectacle, much of the interest will be not just what Biden says in the joint session of Congress, but how whether he avoids serious verbal -- or even physical -- stumbles.

Republican rightwingers like Marjorie Taylor Greene were expected to heckle Biden, as they did last year.

In line with tradition, First Lady Jill Biden will host a number of guests chosen to highlight the White House's priorities.

This year they include a Texan woman forced to leave the state for an abortion, a woman whose IVF treatment was halted by an Alabama court ruling, and the prime minister of Sweden, which became NATO newest member on Thursday.