Nations human rights body, a decade after troops were first deployed against the country's drug gangs.
The lower house of Congress last week passed the long-debated bill that lawmakers say is needed to regulate soldiers used heavily since former President Felipe Calderon sent troops to fight gangs in late 2006.
The United Nations human rights chief on Tuesday called on Mexican lawmakers not to pass the bill, saying Mexico needed a stronger police force. Activists gathered outside the senate to protest what they called the militarization of the country.
U.N. High Commissioner for Human Rights Zeid Ra'ad al-Hussein said the law did not contain strong enough controls to protect civilians from abuses in Mexico, where extrajudicial killings, torture and disappearances are carried out by both security forces and criminal gangs.
'Adopting a new legal framework to regulate the operations of the armed forces in internal security is not the answer. The current draft law risks weakening incentives for the civilian authorities to fully assume their law enforcement roles,' Zeid said in a statement.
Lawmakers are expected to push the bill through committees and later put the measure to a full floor vote. Lower house lawmakers moved quickly on the bill last week after it had languished in committees for years.
The law has broad support from the ruling party and members of the opposition National Action Party. It is strongly opposed by the party of leftist presidential candidate Andres Manuel Lopez Obrador, which has only a handful of senators.
In his presidential campaign, President Enrique Pena Nieto promised to develop a large new national police force, but has ended up relying on military forces for high profile operations.
Zeid said Mexican authorities had told him in 2015 they would decrease the use of the military to replace weak police forces.
The protesters outside the senate carried placards saying the military intervention in crime fighting had failed, with a massive increase in murders over the past 10 years.
The homicide rate in Mexico in 2017 is on track to be the worst on record, with 20,878 murders nationwide in the first 10 months of the year.
Human rights groups and citizens organizations say the military has been guilty of its own abuses while failing to rein in criminal gangs.
They worry the law also opens the door to military intervention in protests, as well as expanding military powers to spy on citizens.
Reporting by Stephanie Nebehay and Michael O'Boyle; Editing by Frank Jack Daniel and Phil Berlowitz.