At least 30 gunmen burst into a drug rehabilitation centre in a Mexican border state capital and opened fire, killing 19 men and wounding four people.

Gunmen also killed 20 people in another drug-plagued northern city.

The killings marked one of the bloodiest weeks in Mexican history and came just weeks after authorities discovered 55 bodies in an abandoned silver mine, presumably victims of the country's drug violence.

The bullet-riddled bodies of 18 men and two women were found yesterday in five different parts of Ciudad Madero, a city in the north-eastern state of Tamaulipas, where violence has surged this year amid a turf battle between the Gulf cartel and its former ally, the Zetas gang of hitmen.

Police had no information on suspects.

It was the deadliest day in Tamaulipas drug violence since 18 gunmen were killed during a series of coordinated attacks on soldiers in April.

Another round of killings occurred late on Thursday night, local time, at the Faith and Life centre in Chihuahua city, about 210 miles south of Ciudad Juarez and the border with El Paso, Texas, state police spokesman Fidel Banuelos said.

A day earlier unidentified attackers killed one man and wounded another at a rehab centre in Ciudad Juarez, which has become one of the world's most deadly cities because of drug violence.

More than 60 people have died in mass shootings at rehab clinics in a little less than two years. Police have said two of Mexico's six major drug cartels are exploiting the centres to recruit hit men and drug smugglers, often threatening to kill those who do not co-operate. Others are killed for failing to pay for drugs or betraying a dealer.

The men at the Faith and Life centre were roused out of bed shortly before 11pm and placed face-down along a hallway, the centre's director Cristian Rey Ramirez said.

Mr Ramirez was alerted to the attack by a telephone call from the centre's pastor.

"He tells me, 'You know what, come here because they just killed everyone'," he said. "There was no warning."

The attackers left messages accusing the victims of being criminals, Mr Banuelos said.

Four other people were taken to hospital, two in a critical condition and two in a serious condition.

Most of the victims were aged from 30 to 40, with some older, and included a blind man, said the Rev Rene Castillo, a minister who gives weekly sermons at the centre, which opened 11 years ago.

"Everyone is so scared now," he said. Violence is "all everyone talks about, especially with all the threats that have been made."

It was the first such attack on the centre, although two men and a woman were kidnapped there in April 2008 while attending a memorial service, Mr Banuelos said.

The three-storey, blue concrete building houses addicts for 90 days, although some of those attacked had been there for up to two years, Mr Castillo said.

President Felipe Calderon, whose war with drug cartels has seen nearly 23,000 people killed since he took office in late 2006, condemned the shootings.

"They are outrageous acts that reinforce the conviction of the need to fight criminal groups who carry out such barbaric acts with full legal force," he said.

The government promised in February to invest £5.3 million in rehab centres and related programmes in Ciudad Juarez. But plans for Mexico's first government-run drug rehab centre have stalled for unknown reasons.

In other suspected drug-related violence, authorities discovered three bodies, one of which had been decapitated, in the Pacific Coast state of Guerrero on Thursday and yesterday.

One of the bodies was found in the town of Iguala accompanied by a note. Police did not reveal what it said.

A second body was found in the town of Tecoanapa leaning against a cement post, its head placed between its knees. Attached to that body was a message written on cardboard whose contents authorities refused to release.

The third body was found with signs of torture in Taxco, a colonial-era tourist town known for its silver jewellery.

In late May, authorities discovered a mass grave in an abandoned silver mine on the outskirts of Taxco that had become a dumping ground for apparent victims of Mexico's drug violence. Authorities found 55 bodies before ending their search last weekend.

Also in Guerrero, two gunmen died Thursday after attacking a military convoy while two brothers, including a 16-year-old boy, died yesterday during an ambush.