Since mid-October, thousands of Central Americans, mostly from Honduras, have traveled north through Mexico toward the United States in a caravan, some walking much of the long trek.

U.S. President Donald Trump has vowed to stop the migrants entering, sending troops to reinforce the border and attempting a procedural change, so far denied by the courts, to require asylum-seekers to remain in Mexico while their cases are heard.

Frustrated and exhausted after weeks of uncertainty, many of the migrants have become desperate since getting stuck in squalid camps in the Mexican border city of Tijuana.

So a number opted to eschew legal procedures and attempt an illegal entry from Tijuana as dusk fell on Monday at a spot about 1,500 feet (450 meters) away from the Pacific Ocean.

In less than an hour, Reuters reporters observed roughly two dozen people climb the approximately 10-foot (3-meter) fence made of thick sheets and pillars of metal. They chose a place in a large overgrown ditch where the fence is slightly lower.

Just before dusk, three thin people squeezed through the fence on the beach and were quickly picked up by the U.S. Border Patrol, witnesses said.

But along the border inland as darkness descended, more and more migrants followed, many bringing children.