The United States has closed its busiest border crossing with Mexico after Mexican police broke up a protest of Central American migrants massed in Tijuana, scattering some demonstrators towards the border where US officers hurled tear gas from the American side.

Traffic in both directions was suspended at the San Ysidro port of entry between San Diego and Tijuana on Sunday (US time), the US Customs and Border Protection agency said on Twitter.

Mexican police, some wielding shields, scattered migrants and protesters, many of whom rushed towards the US border.

US Customs and Border Protection officers repelled them with a volley of canisters as US and Mexican helicopters hovered overhead.

Mexican police clash with the throng of migrants. Photo: Getty

The Mexican government said later on Sunday it had retaken control of the border crossing after nearly 500 migrants tried to cross the US border “in a violent manner” and vowed to immediately deport Central Americans who attempt to enter the United States illegally.

US President Donald Trump has raised alarm for weeks about a caravan of Central American migrants as it approached the United States, with its members planning to apply for asylum on reaching the border.

Tensions rose in recent days as thousands of migrants camped out at a sports complex in Tijuana and hundreds periodically gathered near the main border crossing.

The migrants, who are fleeing poverty and violence, have said they would wait there until they could request asylum despite growing US measures to tighten the border.

Migrants were able to pry open parts of the Mexican border wall. Photo: Getty

On Sunday, hundreds of caravan members were peacefully protesting with chants of “We aren’t criminals! We are hard workers”.

As they reached a bridge and stairway that leads to a square near the US border, they were stopped by Mexican authorities, who told them to wait for permission.

As the morning wore on and frustrations grew, a small group broke off and headed towards a canal between Tijuana and San Diego that leads to the border fence.

Before the group had reached the border, US Customs and Border Protest officers on the other side of the fence launched canisters of what a Reuters photographer identified as tear gas.

Mexico and the US are reportedly still negotiating on how to deal with the flood of migrants. Photo: Getty

After running to relative safety, hundreds of the caravan members held a sit-in. Others continued to approach the border and were repelled by more tear gas.

Trump has deployed military forces to support the Border Patrol and threatened on Saturday to close the entire southern border.

Trump also tweeted on Saturday that migrants at the US-Mexico border would stay in Mexico until their asylum claims were individually approved in US courts but Mexico’s incoming government denied any deal had been struck.