Mexican police have been cracking down on notorious drug gangs over the past few months.<br /><br />Thousands of police have been deployed to strategic locations in what is being called "operation shield" to stop the so-called "cockroach" effect, where a government crackdown in one place forces criminals to scurry to other locations.<br /><br />The head of Mexico City’s security programme is determined to keep violence between cartels and vigilante groups out of the capital.<br /><br />Some Knights Templar cartel leaders have been arrested and accused of killing hundreds of people.<br /><br />Al Jazeera's David Mercer reports from Mexico City.
