Mexico's lower house of Congress early on Saturday passed a bill to give the Army control over the civilian-led National Guard in a controversial move that critics say will unconstitutionally tighten the military's grip over law and order.

The legislative push marks something of a reversal for leftist President Andres Manuel Lopez Obrador, who promised on the campaign trail to deemphasize the military's control over public security but has presided over record violence during nearly four years in office.

The National Guard police force began operating in early 2019 at the behest of Lopez Obrador, who argued the new law enforcement agency would end rampant corruption under its predecessor, the Federal Police.

Opposition legislators and activists have criticized the fledgling force for alleged abuses even as Lopez Obrador extends the military's remit into other areas of civilian life.

"The initiative puts at risk the validity and respect for human rights and goes against the international standards for public security," Amnesty International said before the vote.

Lopez Obrador wanted to enshrine the changes in the constitution, which includes language that public safety must be led by civilians. But facing stiff opposition, he opted to send lawmakers this week a fast-tracked bill needing just a simple majority vote versus a two-thirds super-majority.

If enacted, the legislation would give the Army operational, financial and administrative control of the guard, which currently answers to the civilian-led security ministry.

The bill now moves to the Senate for its consideration, and if passed likely would be challenged as violating the constitution.

In fact, former Supreme Court Judge Diego Valades predicts it would be handily struck down by the top court.

"It's clear it won't prevail," he said, adding that if signed into law it would be challenged and unanimously voted down as unconstitutional.