(Updates with Lopez Obrador speech, Trump tweet)
CIUDAD JUAREZ, Mexico, April 1 (Reuters) - Mexico's presidential front-runner launched his campaign close to the U.S. border on Sunday, demanding respect for Mexicans hours after U.S. President Donald Trump again threatened to scrap a key trade pact and erect a wall between the countries.
Andres Manuel Lopez Obrador signaled that if he won the July 1 election, he would be less accommodating toward Trump than the ruling Institutional Revolutionary Party, or PRI, which has lagged in opinion polls over its failure to contain violence and corruption.
"Mexico and its people will not be the pinata of any foreign government," Lopez Obrador said in a speech to thousands of people, some waving colored flags and balloons. "It's not with walls or use of force that you resolve social problems."
Symbolically, the three-month campaign began in Ciudad Juarez, whose namesake, Benito Juarez, was a 19th century Mexican president from indigenous roots whose exiled government resisted a French colonialist intervention from the unruly city that borders El Paso, Texas.
Trump on Sunday morning reiterated his threat that has hung over the campaign, to cancel the North American Free Trade Agreement among Mexico, Canada and the United States, tying its future to its efforts to stop migrants going north.
"Mexico is doing very little, if not NOTHING, at stopping people from flowing into Mexico through their Southern Border," he said in a tweet.
Lopez Obrador criticized President Enrique Pena Nieto for export-focused economic policies that he said benefited the few, and promised a strategy to build up Mexico's internal markets.
While he reaffirmed his backing for NAFTA, he said talks to renegotiate the agreement should be suspended until after the election, and that any new deal should address wages and immigration.
'EXPERIENCE OF SERVING'
The ruling-party candidate trailing in third place, former Finance Minister Jose Antonio Meade, launched his own campaign on Sunday at the other end of the country in the southeastern town of Merida, touting his years in government as his strength.
"I have the experience of serving and getting results for more than 20 years, without scandals. I live off my salary," Meade said.
Second-place Ricardo Anaya began campaigning on Friday.
Silver-haired Lopez Obrador, a former Mexico City mayor, first ran for president in 2006. He said he would treat Trump's administration with respect but would demand the same for Mexicans.
His unconventional, and at times inconsistent, policy stances have sometimes led to comparisons with the U.S. president.
The 64-year-old candidate played to his leftist base, attacking the current government and saying he would suspend a $13 billion airport project he says is tainted by corruption.
He has promised to "consult the people" on reforms and plans to turn the presidential residence into a cultural center, sell the presidential plane and cut his salary in half.
Sandra Flores, 24, who was at Sunday's rally with her two children, said she and members of her family, including those living in California, supported Lopez Obrador.
"I voted for the president we have now and honestly I'm not satisfied," she said. "(Lopez Obrador) will be able to stand up against what Trump is doing."
(Reporting by Lizbeth Diaz in Ciudad Juarez, Mexico; Writing by Christine Murray; Editing by Frank Jack Daniel and Peter Cooney)