Thousands of Gaza war protesters held a "red line" rally near the White House on Saturday, voicing anger at what they said is US President Joe Biden's tolerance of Israel's bloody military campaign against Hamas.

Chanting "From DC to Palestine, we are the red line," the demonstrators held a long banner scribbled with the names of Palestinians killed by Israeli forces, as the fighting enters its ninth month.

Biden has faced criticism for playing a balancing act on key ally Israel's actions in the conflict.

The White House said in May that a deadly Israeli strike on Rafah did not cross a "red line" that Biden had seemingly set two months earlier when asked about a potential invasion of the southern Gazan city.

"I no longer believe any of the words that Joe Biden says," said protester 25-year-old Zaid Mahdawi from Virginia, whose parents are Palestinian.

"This 'red line' in his rhetoric is rubbish... it shows his hypocrisy and his cowardice," Mahdawi told AFP.

Nursing assistant Tala McKinney, 25, said: "I think we all hope it's going to stop soon but clearly our president is not living up to the words he is speaking to our country. It's outrageous."

The protesters -- almost all wearing red clothing -- held Palestinian flags and signs saying "Biden's red line was a lie" and "Bombing children is not self-defense."

The White House stepped up security with an additional anti-scale perimeter fence ahead of the demonstration, which saw chartered buses ferrying in people from as far afield as Maine and Florida.

Five months from his election battle with Republican candidate Donald Trump, Biden is facing pressure to hang onto Muslim and young voters, blocs seen as crucial to his reelection bid.

"It's very disappointing to have a president who doesn't follow through with their word... I will be voting for a third party," said McKinney.