UPDATE 1-Mexico's leftist and ruling parties declare presidential picks

(Updates with official party nominations and candidate comments)

MEXICO CITY, Feb 18 (Reuters) - Political parties in Mexico formally picked as presidential candidates on Sunday a leftist veteran politician with a strong lead in polls and a former finance minister of the ruling party who has trailed by as much as 20 points.

The ruling Institutional Revolutionary Party (PRI) selected Jose Antonio Meade in front of thousands of cheering party members at a Mexico City stadium often used for rock concerts, ahead of a race set to hinge on vows to end crime and corruption while tackling poverty.

Meade, 48, lags in polls behind rival Andres Manuel Lopez Obrador, who was named a candidate by his leftist Morena party on Sunday in an event at a hotel in the capital.

The right-left coalition leader who holds second place in recent polls, Ricardo Anaya, was due to be nominated later in the day.

Lopez Obrador, a former Mexico City mayor, has held a double-digit lead in most polls since last year and his support grew in two surveys last week.

Speaking in front of several hundred members of the party he founded after his second failed bid for the presidency in 2012, Lopez Obrador, 64, reiterated pledges to combat inequality, corruption and violence.

"We can't let ourselves get used to horror," he told supporters who chanted, "It's an honor to be with Obrador."

He also lashed out against U.S. President Donald Trump's vows to build a wall along the U.S.-Mexico border and said he would appeal to the United Nations for support, signaling a tougher approach to U.S. relations than that of the current government.

"If he insists on building the wall, we're going to turn to the United Nations to defend the rights of Mexicans," he said. "I'm conscious of my historic responsibility."

Meade, meanwhile, pledged to crack down on crime and impunity, issues that have dogged President Enrique Pena Nieto's administration and hurt his party's credibility.

"They're asking us to take away guns, money and property from criminals and those who are corrupt, and that's exactly what we're going to do," he said.

Delegates and labor unionists clad in the party's signature crimson sang "Let's go with Pepe," referring to the nickname for "Jose."

Meade has lost support in recent polls as he struggles against public anger at the government over violence and corruption scandals.

With six weeks to go before campaigning begins for the July 1 vote, his campaign says early polls have traditionally been unreliable indicators of final results in Mexico.

Meade also faces competition from Anaya, a young leader of a right-left coalition who has narrowed the gap with Lopez Obrador in recent polls.

The youngest of the three at 38, Anaya resigned in December from his post as president of the center-right National Action Party (PAN) to pursue the presidency in an alliance with the center-left Party of the Democratic Revolution (PRD).

His coalition backs a universal basic income program to fight poverty.

"If this trend continues, the race will become more competitive between the two front runners," political risk analysts at Eurasia Group said in a note on Friday. "But voter intentions have historically shifted throughout the campaign season, so much could change."

Analysts have said Lopez Obrador could slow the pace of Pena Nieto's opening of the state-run energy sector to private investment, but they expect he would face a divided Congress that would make any rapid shifts in policy unlikely.

Still, some international investors are concerned he could undermine decades of free-market reforms in Mexico.

The election will mark the first time independent candidates can run for the presidency. Margarita Zavala, wife of former President Felipe Calderon from the PAN, and Jaime Rodriguez Calderon, who became Mexico's first independent governor in 2015, are expected to get enough signatures to run.

Analysts doubt any of the potential independent candidates have a chance, but their supporters could be a factor in the outcome in a close race. (Reporting by Diego Ore and Michael O'Boyle; Writing by Daina Beth Solomon; Editing by Frank Jack Daniel and Phil Berlowitz)