Its organiser says around 7,000 people now make up this migrant caravan, up from 5,000 just days ago.
Most of them are from Central and South America, local aid groups say.
US President Joe Biden is due to meet leaders from the region on Friday to discuss how to curb the flow of migrants to the United States.
President Biden has come under attack for his handling of migration, in particular on the US-Mexico border.
Republicans say he is not doing enough to stem the flow and Democrat mayors, whose cities have seen an influx after Republican governors bussed newly arrived migrants there, have joined in the criticism, saying that their cities cannot house and feed them.
The number of people apprehended at the US's southern border exceeded 2 million both in the 2022 and the 2023 fiscal years.
In September 2023 alone, US Border Patrol apprehended more than 200,000 migrants crossing the US-Mexico border unlawfully, according to US Homeland Security figures.
Those who have joined the migrant caravan, which left the city of Tapachula on Monday, say they are determined to make it to the US.
"In Venezuela things are very tough, we can't live with the money we get, it's not enough for us, and that's why we're going to the United States," a Venezuelan father who was embarking on the long walk with his wife and two daughters told Reuters news agency.
One of the organisers of the caravan, Irineo MÃºjica of the NGO People without Borders, said that the migrants had set off together after having been left "stranded" by the Mexican authorities in increasingly dangerous Chiapas state.
"Organised crime is already taking over Chiapas, violence is everywhere. We are trying to save lives with these kinds of actions," he said of the caravan.
A migrant from Honduras told Mexican newspaper La Jornada that he decided to join the trek north after having waited in vain for a transit permit to cross Mexico.
"I could not keep on waiting without money, sleeping in the street, that's not a life. Better that we should head up [northwards] and let's hope the [Mexican] government helps us, doesn't stop us".
Some previous caravans have clashed with Mexican police, which tried to stop them from walking along major highways.
A group of thousands of migrants who have set off on foot for the United States from southern Mexico is steadily growing in size.