The US president said the thousands of Central American migrants travelling towards America would have to apply for asylum in Mexico first.
He tweeted: "Full efforts are being made to stop the onslaught of illegal aliens from crossing our Souther(n) Border.
"People have to apply for asylum in Mexico first, and if they fail to do that, the U.S. will turn them away. The courts are asking the U.S. to do things that are not doable!"
In a second tweet, he added: "The Caravans are a disgrace to the Democrat Party. Change the immigration laws NOW!"
The US president said the thousands of Central American migrants travelling towards America would have to apply for asylum in Mexico first.
He tweeted: "Full efforts are being made to stop the onslaught of illegal aliens from crossing our Souther(n) Border.
"People have to apply for asylum in Mexico first, and if they fail to do that, the U.S. will turn them away. The courts are asking the U.S. to do things that are not doable!"
In a second tweet, he added: "The Caravans are a disgrace to the Democrat Party. Change the immigration laws NOW!"
It was not immediately clear where the additional travellers had materialised from since about 2,000 gathered on the Mexican side on Saturday night.
They are thought to be people who had been waiting on the bridge over the Suchiate River or in the Guatemalan town of Tecun Uman and who decided to cross during the night.
As they passed through Mexican villages on the outskirts of the city of Ciudad Hidalgo, they drew applause, cheers and donations of food and clothing from Mexicans.
Despite warnings to turn back this week from US President Donald Trump, one migrant, Erasmo Duarte, from Honduras, said before the journey: "We are going to reach the United States."
Mr Trump threatened to deploy soldiers to the border with Mexico and close it off to stop the "large caravans" coming to America.
He has sought to make the border security with Mexico a campaign issue around two weeks before midterm elections. His promise to build a wall along the border was also one of his key policies during his election campaign.
On Friday he told reporters those travelling to the US en masse were not "little angels" but "hardened criminals".
He said: "I don't want them in our country. And neither does our country want them in our country."
As a military helicopter circled overhead, migrants who said they were fleeing a toxic mix of violence, poverty and
corruption in Central America wondered if police would seek to turn the caravan back.
Authorities handed out numbers for people to be processed in a strategy seen before at US border posts when dealing with large numbers of migrants.
But many became impatient before circumventing the border gate and swarming across the river on rafts or swimming in full view of hundreds of Mexican police manning the blockade on the bridge.