(Updates with comment from Mexican president)

By Delphine Schrank

TECUN UMAN, Guatemala, Oct 19 (Reuters) - Hundreds of people in a caravan of migrants that crossed from Honduras into Guatemala staged a dramatic bid to breach the Mexican border on Friday, as local governments began preparing to disperse the convoy under pressure from Washington.

Migrants poured through Guatemalan border posts in the town of Tecun Uman and onto a bridge leading to Mexico, only to be halted by dozens of Mexican police in riot gear. Mexico's president sharply rebuked the migrants for the border surge.

U.S. President Donald Trump has warned the Central American caravan must be stopped before it reaches the United States, and Honduras and Guatemala said late on Friday they were mobilizing to assist the return of Honduran migrants to their homeland.

Some migrants violently shook fences at the border and police said a few officers were injured in clashes. A handful of migrants jumped into the Suchiate River below to swim for rafts. Others turned back toward Guatemala.

Carrying backpacks and small children, many bedraggled migrants simply sat down on the bridge. Some said that they had been teargassed. As the afternoon drew on, a tropical storm, Vicente, formed nearby off the Pacific coast.

Jose Brian Guerrero, a 24-year-old Honduran traveling with neighbors and his extended family, said he had joined the caravan to escape violent street gangs, and to find work.

"There's nothing for us in our country," said Guerrero, who used to sell beans in Honduras.

On Friday evening, Honduran President Juan Orlando Hernandez said he had spoken to his Guatemalan counterpart Jimmy Morales for clearance to send civil protection personnel to help the Hondurans and to find transport for those wanting to return.

"We'll continue this operation for as long as is necessary," Hernandez said in a post on Twitter.

Shortly afterwards, Guatemala's government tweeted that Hernandez would meet Morales on Saturday in Guatemala City to implement a strategy for returning the Honduran migrants.

Meanwhile, in a late televised address, Mexican President Enrique Pena Nieto called the mass rush to penetrate the border "unprecedented", accusing some migrants of attacking police.

"Mexico does not and will not allow (people) to enter its territory illegally, let alone violently," he said.

A similar caravan of Central Americans that formed in southern Mexico in late March also drew the ire of Trump, who on Thursday threatened to use the military and close the southern border if Mexico did not halt the new march.

Such a move would cause chaos on the crossing, one of the world's busiest, and badly disrupt trade.

Speaking in Scottsdale, Arizona on Friday, Trump said he "appreciated very much" Mexico's efforts to stop the caravan.

"If that doesn't work out, we're calling up the military - not the (National) Guard - we're calling up the military," he told reporters. "They're not coming into this country."

Trump has also threatened to cut off aid to Honduras, Guatemala and El Salvador - some of the poorest and most violent countries in the Americas - if they fail to prevent undocumented immigrants from heading to the United States.

Their emigrants make up the bulk of people now caught trying to enter the United States illegally every year.

Several migrants at the Guatemala-Mexico border spoke of entire neighborhoods leaving their homes to join the trek after news circulated on social media of a call for a new "caravan" to Mexico six months after the previous one.

U.N. ASSISTANCE

Earlier, Mexican Foreign Minister Luis Videgaray and U.S. Secretary of State Mike Pompeo met in Mexico City and discussed the caravan, which set off from Honduras last weekend.

"It's a challenge that Mexico is facing, and that's how I expressed it to Secretary Pompeo," Videgaray told a joint news conference.

Pompeo said he and Videgaray spoke of the importance of stopping the caravan before it reaches the U.S. border.

In contrast to the earlier caravan, which had advanced into Mexico before officials began intensive efforts to process the migrants, the Mexican government turned its attention to the new group right on its southern border.

Mexico's government has sought assistance from the United Nations High Commissioner for Refugees (UNHCR). As Mexico processes the migrants, the caravan will likely slowly disperse.

On Friday morning, Videgaray said the caravan had close to 4,000 people and that the migrants could individually present their claims to enter Mexico or seek refugee status.

"We haven't had a caravan or group of this size seeking refuge at the same time, that's why we've sought the support of the United Nations," he told Mexican television.

Mexico says the migrants without a legitimate case to claim refuge in Mexico will be returned to their countries of origin. A Mexican official, speaking on condition of anonymity, said the country had the capacity to process around 200 people a day.

POLICE WAITING

Hundreds of Mexican police were sent to guard the border between the Guatemalan town of Tecun Uman and Ciudad Hidalgo in Mexico to prepare for the migrant caravan's arrival.

Manelich Castilla, the head of Mexico's federal police, said at the scene that his officers had restored order after the rush of migrants towards the border, and would begin allowing people to be processed in an orderly fashion.

Six police had been injured, Castilla said.

UNHCR spokesman Charlie Yaxley said the agency was reinforcing capacity in southern Mexico to offer counseling, legal assistance and humanitarian aid to asylum-seekers.

"UNHCR is concerned that the mobilization of such a large number of people in a single group will overwhelm the capacities that exist in the region," he told a news conference. (Reporting by Delphine Schrank in Tecun Uman; additional reporting by Veronica Gomez, Julia Love, Daina Beth Solomon, Noe Torres and Dave Graham in Mexico City, Tom Miles in Geneva and Edgard Garrido in Tecun Uman Writing by Dave Graham Editing by Rosalba O'Brien and Sandra Maler)

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