Families in the migrant caravan celebrated the breach of a gate on the Guatemala-Mexico border. Getty Images
Hundreds of migrants travelling across Central America in a mass caravan to the US have tried to breach Mexico's southern border and enter the country.
Some migrants broke through Guatemalan border fences but then clashed with Mexican riot police in no man's land.
The migrants, mostly from Honduras, say they are fleeing violence and poverty, and include women and children.
President Trump, who has threatened to close down the US border, said the military would be called upon if needed.
"They might as well turn back, they're not coming into this country," he told reporters on Friday.
After talks on border security in Mexico, Secretary of State Mike Pompeo said the situation was reaching "a moment of crisis".
What happened on the border?
Hundreds of Central American migrants broke through barriers on a bridge which crosses the river border between Guatemala and Mexico.
Dozens of Mexican police in riot gear reportedly fired tear gas to force them to retreat into no-man's land after being attacked with stones.
A number of migrants jumped into the Suchiate river to reach rafts, while others either turned back towards Guatemala or simply sat down on the bridge.
The Mexican authorities have told migrants, who include women, children and old people, they want an orderly process and only those with valid documents will be allowed in.
On Thursday, Mr Trump thanked Mexico in a tweet for the decision to send hundreds of police officers to meet the migrant caravan at the border.
Mr Trump has also threatened to cut aid to countries allowing the caravan to pass.