By Natalie Schachar

MEXICO CITY (Reuters) - Mexican billionaire Carlos Slim said on Friday that Donald Trump's plans would "destroy" the United States economy and joked that the Republican presidential candidate's proposed wall at the border would be undermined by smuggling tunnels.

Slim made his first public comments since Trump accused him of trying to help the candidate's Democratic opponent, Hillary Clinton, in the U.S. election at an event in Mexico City.

Slim said that he thought it was unlikely Trump would win the election on Tuesday, and dismissed the New York businessman's economic proposals.

"If they slap on a 35 percent (tariff) it would destroy the American economy," he said at an event in Mexico City, referring to Trump's threat to put a 35 percent tax on many goods manufactured in Mexico and shipped to the United States.

The comments appeared to pit Slim, who for several years was the richest man in the world, against Trump, who has said his business acumen and experience as a real-estate developer make him the best candidate for the White House.

In an October speech, Trump accused Slim, the top shareholder in The New York Times Co <NYT.N>, of trying to help Clinton through his stake.

The Sulzberger family controls the company's voting shares; Slim's shares have limited voting rights.

Slim, asked about Trump's comments on the sidelines of the Mexico City event, said, "I don't even know Trump, the U.S. elections have to be decided by the population, the people of the United States and, to be honest, the personal life of Trump doesn't interest me."

Trump has said he would renegotiate or tear up a key trade deal with Mexico and Canada and build a massive border wall, and has attacked U.S. companies investing there, battering Mexico's peso.

"It's not easy to build a wall, anyway they build tunnels and most people arrive by plane," Slim joked, referring to drug cartels digging tunnels beneath the border to smuggle their products into the United States.

(Reporting by Natalie Schachar; Writing by Christine Murray and Joanna Zuckerman Bernstein; Editing by Jonathan Oatis)