Former Mexican President Vincente Fox has accompanied news of Donald Trump's surprise visit to Mexico City with an extended tirade against the GOP nominee, and has accused the Republican candidate of being obsessed with Hillary Clinton.

Trump has demonstrated to American voters and the world that he is "extremely weak," the former head of state said in an interview this week with Time magazine.

"His obsession [and] his very unmanly behavior with his opponent Hillary [Clinton]. That's not the way to show that you're presidential," Fox said.

"It's incredible that we have run so low, so vulgar, and all of this has been provoked by him. He came like the bad guy, like the street fighter, like the gang member to break everybody who stands in front of him," he said.

"This nation is so great, so powerful, it's such a leader. Why can Trump tell people that he's going to make America great? That poor guy with such poor thinking, how can he make America great?"

Fox's remarks came just hours before Trump announced late Tuesday evening that he had agreed to travel to Mexico City to meet personally with Mexican President Enrique Pena Nieto.

The meeting Wednesday afternoon between Trump and Nieto, who initiated the meet-up, is scheduled to take place ahead of the GOP nominee's major address in Phoenix on immigration reform.

As Trump's team finalized its plans for the candidate to fly to Mexico City, Fox told Time that there is nothing admirable about the billionaire businessman, "not even his hair."

"What can you admire about a person like this? You cannot admire anything about him, he's such a poor human being that I don't see any qualities in him. So no admiration for him at all," Fox said.

Asked whether it was in good taste for a former head of state to rail against a presidential candidate, as the former Mexican president has done since Trump first announced his candidacy in June 2015, Fox's answer was straightforward.

"Of course, of course. It's key and crucial to speak the truth, to communicate the truth. And I speak the way I feel, the way I perceive, the way I think. Donald, his aggressiveness has come to the world's attention," he said.

"For us in Mexico, we consider him an absolute danger, trying to break and divide what we have built for so long in the border, in the relationship of our two nations ... So one part of my reaction has to do with combating his aggressiveness, his discrimination. The other part has to do with his ignorance," Fox said.

Later, after Trump's team had confirmed his trip to Mexico City, the GOP nominee and Fox sparred Wednesday morning on social media, as the latter accused the former of lying about a previous invitation to visit Mexico.

Fox also took his anti-Trump message to CNN Wednesday morning, where he told anchor Chris Cuomo that the Republican candidate is using Mexico for his own political gain.

"I think it's nothing more than a political stunt. Trump is using Mexico, he's using President Pena to boost his sinking poll numbers," he said