Some of the victims had their hands tied and showed signs of having been tortured, reports said.

Veracruz, on the Gulf of Mexico coast, has seen a spike in drug-related violence in recent months.

On Tuesday afternoon, people began to tweet that men wearing military-style uniforms were blocking the underpass near a big shopping centre, pointing their guns at motorists and dumping the bodies.

"They don't seem to be soldiers or police," a tweet read, according to Associated Press. Another said: "Don't go through that area, there is danger."

Veracruz Attorney General Reynaldo Escobar said the corpses - 23 men and 12 women - were found in two vehicles. Of the seven victims so far identified, all had criminal records, he said.

"We have begun the corresponding investigations and have identified some of the bodies and have confirmed that they all had prior criminal records and were involved with organised crime, like kidnapping, extortion, homicide, among other crimes."

Mr Escobar added: "We have never seen a situation like this before."

On Monday, 32 prisoners escaped from three jails in Veracruz, but Mr Escobar said there was no indication that any of the inmates were among those killed.

Local media reported that some had been found with their hands bound and appeared to have been tortured.

Veracruz had been spared much of the drug-related crime that has afflicted other regions of Mexico, but has recently witnessed an escalation of violence.

The upsurge has been blamed on a struggle between the Zetas and its rivals in the Gulf Cartel for control of drug smuggling routes.

Security expert Alberto Islas said drug gangs were looking to gain control of the area as they broadened their global reach.

"Veracruz is an important strategic port. That has always been true for trade and commerce, but it's now also true for drug exports," Mr Islas told Reuters news agency.

More than 40,000 people have been killed in Mexico since President Felipe Calderon declared war on the drugs cartels in 2006.