US Democrats on Thursday ended a sit-in protest on the floor of the House of Representatives after more than 24 hours, pledging to continue the fight for tighter gun controls in the wake of the Orlando massacre.

"By sitting in, we are really standing up for America," said veteran Georgia congressman John Lewis, an icon of the US civil rights movement, as he ended the protest. He later marched down the steps of the US Capitol with other Democrats.

"We must never give up or give in. We must keep the faith and we must come back here on July 5 more determined than ever before."