A demonstration against Mexico's 20% petrol price rise turned violent after a lone protester drove his truck into a line of police guarding a fuel distribution terminal in Baja California.
P olice said four officers were injured in the incident on Saturday in Rosarito, near the border city of Tijuana.
Video showed the small pick-up driving straight into the line of riot police, then backing up and speeding off.
Largely peaceful protests against the fuel price increases continued elsewhere in Mexico on Saturday, and looting seen earlier in the week largely subsided. But nervousness remained.
Officials in Veracruz, one of the states hardest-hit by the looting on Wednesday and Thursday, said some neighbourhood groups had begun to form patrols of residents armed with staves or machetes to ward off looters.
Veracruz Governor Miguel Angel Yunes Linares said 532 people had been detained in his state alone and that rumours of further looting - apparently unfounded - had caused "an artificial psychosis".
He said that in the northern part of the state "the neighbours decided to arm themselves with staves, machetes, creating the impression that there were armed groups of criminals".
Mr Yunes Linares said the government was trying to convince residents to stop such patrolling.
The interior department reported a total of more than 1,500 people have been detained for looting or disturbances nationwide since protests began early in the week.
It is unclear how many have been charged. Hundreds of stores were looted, mainly on Wednesday and Thursday. Police protection of shops has been stepped up since.
The police reported continued protests, and some highway blockages, on Saturday.
Thousands of people marched down main avenues in the western city of Guadalajara to protest against the increases, which are part of a government effort to deregulate fuel prices.
Despite persistent rumours that political interests might have egged on the looters to smear the petrol-hike protesters, Mr Yunes Linares said there was "no evidence that political parties were involved".
He said the authorities were investigating whether criminal gangs had taken part.