Fifteen people were injured, some in the stampede, after at least one shooter opened fire before dawn at the Blue Parrot nightclub during the BPM festival, Mayor Cristina Torres told Turquesa radio.
Three of the dead are foreigners, said Torres, mayor of the municipality of Solidaridad, which includes the popular beach resort.
"For the moment we have indications that one person opened fire," Torres said, adding that authorities were contacting the US, Canadian and Colombian consulates to provide information about their citizens' conditions.
Playa del Carmen is 68 kilometers south of Cancun. The region, popular among American and European tourists, has been largely spared from the drug violence plaguing other parts of Mexico.
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An editor for the London-based music magazine Mixmag who was in the backstage area of the Blue Parrot said at least four or five shots were fired at around 2:45 (local time) or 3 am, indicating that it happened in front of the club.
"Security guards at first didn't think it was shots and kept claiming it was fireworks, saying everything was OK. Then people kept running and said they saw a gun. We kept hiding until they opened the back gate and we ran outside," Lee said.
Lee wrote on Twitter that the music was still playing five minutes after the shots were heard. The music, she said, was "super loud, likely those inside didn't even hear."
"Bodies on the ground. Didn't seem like shooters tried to enter the party," Lee wrote.
"Police took over 10 minutes to arrive. Victims likely include tourists/party goers, poss (possibly) a security guard."
Images shared on social media showed people cowering or running down the street.
The BPM Festival issued a statement on Facebook saying the attack was perpetrated by a lone shooter in front of the club.
"The violence began on 12th Street in front of the club and three members of the BPM security team were among those whose lives were lost while trying to protect patrons inside the venue," the statement said.
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