Mexico has amassed 250 extra police on its border with Guatemala in preparation for the arrival of a migrant caravan heading for the United States, officials say.
The caravan has been growing steadily since it left the violent Honduran city of San Pedro Sula on Saturday and is estimated to include between 1500 and 4000 people.
The migrants hope to reach Mexico and then cross its northern border with the US, to seek refuge from endemic violence and poverty in Central America.
US President Donald Trump has criticised the caravan and has already threatened Honduras, Guatemala and El Salvador with aid cuts if they fail to stop migrants from crossing their borders on the way to the US.
"Hard to believe that with thousands of people from South of the Border, walking unimpeded toward our country in the form of large Caravans, that the Democrats won't approve legislation that will allow laws for the protection of our country," Trump said on Twitter on Wednesday.
The first migrants have arrived in the Guatemalan border town of Tecun Uman, according to the country's Bishops' Conference.
The migrants, who include children and elderly people, are moving by bus, other vehicles and on foot. Guatemalan authorities pledged to guarantee order and security, but let the migrants pass through.
Mexico has announced that it will not allow the entry of people without the necessary documents. However, the migrants have the possibility of seeking asylum in Mexico, Amnesty International said.
Mexican police chief Manelich Castilla said earlier that police would assist the migration authority, but that its task was not to block the caravan.
Meanwhile, about 500 people have left Choluteca department in southern Honduras to join the caravan. They were unable to stay in their country because of high crime rates, unemployment and economic difficulties, one of the organisers told local media.
Honduran President Juan Orlando Hernandez has called on Hondurans not to risk their lives by participating in the caravan.
"Today you are exposed to criminal gangs, traffickers of people and organs, drug traffickers ... criminal gangs," Hernandez said.
Australian Associated Press