The attackers, who came in two pick-up trucks and one car, stormed into the bar "El Sabino Gordo" and opened fire on the patrons, they said.[break]
"Up to now, we have found 17 bodies inside the bar and three more outside," an official from the prosecutor´s office told AFP on condition of anonymity.
The assailants numbered at least 15, according to one local resident. They also strafed a hot dog stand with three people standing in front of it.
The bar is located on Villagran Street, in the center of a busy nightlife district.
Monterrey has been the site of a bloody confrontation among leading Mexican drug cartels feuding over lucrative narcotics supply routes leading to the United States.
Earlier Friday, 11 bodies with gunshot wounds were found in Chalco, a suburb of the Mexican capital, police said.
Mexico has been convulsed by a massive crime wave, with some 37,000 people killed in battles among rival drug cartels and between traffickers and security forces since 2006, when the government launched a military crackdown.
The National Commission for Human Rights (CNDH) said in a report on Wednesday that it had documented some 523 human rights violations since 2005, including murders, attacks, kidnappings and arbitrary arrests.
23 killed in attack on bar in southern Mexico