The northern Mexican border city of Nuevo Laredo saw a brutal day of gang violence on Friday, with 14 headless bodies found stuffed in a vehicle and nine bodies found hanging from a bridge.

The gruesome crimes came less than two months before Mexico's presidential election, and just ahead of a key weekend debate between the leading candidates, during which security policy is likely to be a key issue.

Horrified motorists in Nuevo Laredo - across the river border from Laredo, Texas - came upon the blood-stained bodies of four women and five men hanging off a bridge, along with an apparent message from a drug gang.

Police then discovered the 14 headless bodies in a vehicle parked in front of the Association of Customs Agents on one of the city's main avenues. The 14 heads were found in ice boxes outside the city hall.

The grim spectacles were extreme even for Nuevo Laredo, a city of nearly 400,000 in the northeastern state of Tamaulipas, which has seen some of the most gruesome episodes in Mexico's brutal five-and-a-half year drug war.

State security forces and soldiers cordoned off the areas where the bodies were found and made no immediate comment.

Nuevo Laredo is regularly the scene of vicious disputes between the Zetas drug gang - set up in the 1990s by Mexican ex-elite soldiers - and their former employers, the Gulf cartel, now believed to be allied to the Sinaloa cartel of billionaire fugitive Joaquin 'El Chapo' Guzman.

The city is a key site for smuggling illegal narcotics into the United States: around 40 per cent of the land cargo heading north, much of it from the industrial city of Monterrey, funnels through Nuevo Laredo.

In April, the dismembered remains of 14 men were found inside a van left near Nuevo Laredo city hall. Days later a car exploded outside police headquarters.

More than 50,000 people have been killed in Mexico's war on drugs since December 2006, when outgoing President Felipe Calderon launched a nationwide military crackdown on organized crime.

Most of the deaths have been from turf battles between rival gangs.

In the northwestern state of Sinaloa, 34 people were killed over the past seven days in clashes between soldiers and drug gangs.

In neighboring Veracruz state, further south on the Gulf of Mexico, security forces on Thursday found the dismembered bodies of two missing news photographers and two others, just days after a magazine reporter was killed in the same state.

The photographers, Gabriel Huge and his nephew Guillermo Luna, were buried in Veracruz city on Friday.

As in Tamaulipas, many crime reporters have fled Veracruz in recent months amid threats and drug gang battles.

Hundreds of journalists demonstrated in cities across Mexico on Friday, calling for better protection for reporters and denouncing the failure to punish those responsible for similar murders.

"Not One More," read placards carried by protesters in Mexico City.

The four leading candidates vying to succeed Calderon will square off in a debate on Sunday, ahead of the July 1 election.

Even though security is the top concern among voters, the candidates have not addressed the issue unless directly asked, said Vicente Sanchez, a professor at the School of the Northern Border.

"They have given an occasional outline, but have not gone deep into what they would do or how they would do it," he said.

The recent wave of violence could force them to "present a more clear position and face the issue in a more open way," Sanchez said.