TECUN UMAN (Reuters) - Hundreds of Central Americans gathered on the Guatemala-Mexico border on Sunday, aiming to cross en masse early on Monday in what could prove a stiff test of the Mexican government's pledge to satisfy U.S. demands to curb migrant flows.

President Donald Trump has threatened to hurt Mexico and Central American countries economically if they allow large numbers to reach the U.S. border. U.S. border agents have tracked the latest exodus from Honduras this week.

Small groups of migrants crossed the bridge from Tecun Uman, Guatemala, to Ciudad Hidalgo, Mexico, on Saturday after Mexican National Guardsmen repelled an attempt by some of the Central Americans to force their way through the border gates.

Afterwards, most of the migrants withdrew to Tecun Uman to await the arrival of more travellers behind them in the hope they stand a better chance of progressing in one caravan.

"We've been getting organized," said Jose Luis Cruz, a 33-year-old autoworker from Comayagua, Honduras, who left behind a wife and two children in a bid to reach the United States.

Cruz said migrants waiting in Tecun Uman wanted to try and enter Mexico en masse at around 5 a.m. on Monday, hoping that authorities would let them pass to avoid clashes.

"If we cross alone, they'll get us, and all the sacrifices we've made to get here will have been in vain," he added.

Guatemala's government says at least 4,000 people have entered from Honduras since Wednesday, making for one of the biggest surges since three Central American governments signed agreements with the Trump administration obliging them to assume more of the responsibility of dealing with migrants.

Tecun Uman's main migrant shelter quickly filled to capacity with 600 people, and another 1,300 broke their journey in a second, makeshift refuge, said local priest Alfredo Camarena.

Mexican authorities have so far controlled the border more successfully than in late 2018, when a large caravan of people sought to break through at the same crossing. Many later crossed into Mexico via the Suchiate River dividing the two countries.

Mexico has offered migrants work in the south, but those who do not accept it or seek asylum will not be issued safe conduct passes to the United States, the interior ministry said.