You knew it had to happen. As soon as the 30-some regional leaders went behind closed doors at the so-called Summit of Unity in Cancun, Mexico, the fat hit the fire.

Venezuela's leftist president, Hugo Chavez, and his right-wing Colombian counterpart, Alvaro Uribe, engaged in a nasty verbal row during the two-day summit of Latin American and Caribbean leaders.

"Be a man!" Uribe shouted at Chavez, according to Mexican press accounts. Chavez accused Uribe of plotting his assassination.

It took two other ideological opposites, Mexican President Felipe Calderon, the summit host, and Cuban President Raul Castro, to calm the two chafing leaders.

Tensions have been high between Venezuela and Colombia for a while, and worsened when Uribe's government agreed to let the U.S. operate military bases on Colombian territory. And Bogota accuses Venezuela of harboring leftist anti-government guerrillas.

The summit, meanwhile, ended Tuesday with agreement on the formation of a new regional bloc of Latin and Caribbean countries that excludes the U.S. and Canada.

And Milenio newspaper reported that despite the extraordinary security measures employed to protect the summit, purported drug traffickers managed to leave signs of protest on key Cancun streets.

-- Tracy Wilkinson in Mexico City

Top photo: Hugo Chavez, left, and Alvaro Uribe confer in a file photo. Credit: Getty Images.

Lower photo: A painted message, purported to be from drug traffickers, left outside the Cancun summit venue. Credit: Milenio.