Unidentified men on foot threw the grenade from the edge of a square where people had gathered on a warm autumn night in the municipality of Guadalupe, which is part of Monterrey, police said. Four children were among the injured.

"There was a loud explosion and people started screaming and running," a witness, who declined to give her name, told local radio.

The explosion was the fourth from a grenade during the weekend in Monterrey, one of Latin America's premier business cities. No one was injured in the earlier attacks.

Television images from Saturday's scene showed crying children rushed to the hospital, their heads wrapped with white bandages, but no one was seriously injured, Guadalupe Mayor Ivonne Alvarez told local newspaper El Norte.

Mexico's Interior Ministry condemned the incident and promised a crackdown. It blamed organized crime for the grenade attack in a reference to drug cartels vying for smuggling routes into the United States and Mexico's lucrative home-grown drug markets.

"These actions underline the need for the three levels of government... to face head on the threat from organized crime," the ministry said in a statement.

Monterrey, which has close U.S. business ties, had been an oasis of calm but has been sucked into Mexico's drug war since the start of this year. One of the three grenade attacks on Friday across Monterrey came in the center of the city near the U.S. consulate, which had been targeted at least twice with grenades in 2009.

The government blames a split between the powerful Gulf cartel and its former armed wing, the Zetas, for much of the violence, which also has surged in neighboring Tamaulipas since the start of the year.

In September 2008, a drug gang killed eight revelers in a grenade attack on Independence Day celebrations in the city of Morelia in western Mexico, raising the specter of what the Mexican media called "narco-terrorism."

But fears of a surge in attacks on civilians have not materialized across Mexico and most of the dead in the drug war are hitmen, corrupt police and soldiers, although civilians have been caught in the cross-fire of deadly shootouts.

More than 29,000 people have died in drug violence since President Felipe Calderon launched his army-led offensive on drug cartels in late 2006.

The explosion in Guadalupe is likely part of a wider intimidation campaign by drug gangs across northern Mexico. Drug gangs detonated several bombs in cars in Tamaulipas state earlier this year but no one was injured in the blasts.

While the drug war has become notorious for grisly murders, Mexico has yet to reach the level of violence of Colombia in the 1980s and early '90s, when drug smugglers set off powerful car bombs and also bombed a commercial airliner, killing 107 people, in protest at extraditions to the United States.

Source: Reuters