Republican presidential candidate Donald Trump campaigns on a vague immigration plan centered on fortifying the U.S.-Mexico wall and deporting some, if not all, of the undocumented immigrants living in the country.

Trump's efforts over the last 14 months come to a head Wednesday as he flies to Mexico for a meeting with President Enrique Peña Nieto before delivering a major immigration speech in Phoenix.

The irony is not lost of former Mexico President Vicente Fox, who apologized for the meeting on behalf of the Mexican people Wednesday morning.

"It is a very opportunistic move, and I hope U.S. public opinion, U.S. citizens can see this and finally, and finally see what is behind Trump, this false prophet that is just cheating everybody," Fox said on CNN's "New Day."

Fox said Mexicans "don't like him; we don't want him; we reject his message," adding that the only way the visit makes sense is if Trump apologizes for his anti-Mexican rhetoric.

Trump and Fox's Twitter War

Fox was among the first Mexican citizens to disavow the Trump-Peña Nieto meeting, tweeting that there was "no turning back."

"Trump, your offensive behavior towards Mexicans, Muslims and others have put you in the hole you're now in. Adios, Trump!," Fox wrote.

Shortly after Fox's CNN interview, Trump replied by tweeting Fox extended a similar invitation. "Former President Vicente Fox, who is railing against my visit to Mexico today, also invited me when he apologized for using the "f bomb," Trump wrote.

Last February, Fox told Fusion's Jorge Ramos that Mexico is "not going to pay for that f-----g wall." Fox backtracked two months later on Fox News' "The O'Reilly Factor," inviting Trump "to learn about the real Mexico." Fox was less sympathetic upon learning about Wednesday's visit, calling on Trump to "show some respect."

Mexican Twitter Users Scold Peña Nieto

In the U.S., reaction to Trump's last-second trip is of wonder and amusement, given that the New York businessman is visiting a nation he condemns for sending "criminals" and "rapists." Aside from musing over a deportation task force, Trump has revealed little of what a Trump presidency would mean to some 11 million undocumented immigrants.

Mexicans, including many politicians, were alarmed.

Democratic Revolution Party leader Miguel Barbosa, one of Peña Nieto's ardent critics, tweeted that it was a "grave political mistake" to let Trump serve his own interests. Former President Felipe Calderón's wife, Margarita Zavala, said "Mexicans have dignity and repudiate his hate speech," and historian Hector Aguilar Camin said the only alternative to Trump apologizing is for Peña Nieto to lose the little support he still has.

Former Mexican diplomats and ambassadors to the U.S. also tweeted their disapproval, including Arturo Sarukhan, who tweeted "Trump & his xenophobia & sends the message that there is not cost bashing Mexico & Mexican migrants."

Social media users tried planning a protest at El ángel in downtown Mexico City, but it was short-lived. Other voiced opposition with hashtags like #NoEresBienvendioTrump (Trump, You Are Not Welcome) and #FueraTrumpFueraEPN (Out with Trump, Out with Enrique Peña Nieto).