MEXICO CITY (Reuters) -Mexican senators on Thursday passed regulations stipulating how a judicial overhaul that reforms the country's constitution will be implemented to popularly elect judges.

Congress passed a judicial reform in September which calls for the election of judges by popular vote, a move proponents say will ensure the judiciary serves the people and not organized crime but which critics say does not adequately target those they believe to be behind high rates of corruption.

Pushed by former President Andres Manuel Lopez Obrador in the last weeks of his administration, the judicial reform also spooked investors and strained relations with allies.

The secondary legislation passed on Thursday will now go to the lower house for discussion and vote. Ruling party Morena and its allies have a large majority in the chamber.

The proposal passed the senate with 81 of 128 votes in favor.

Before Oct. 16, the Senate must call for an extraordinary election to be held in mid-2025, to elect justices. The election would replace all Supreme Court justices, whose numbers will be reduced from 11 to nine.

Other top courts, regional chambers of the electoral court and half of magistrates and district judges will also see their positions up for election.

In 2027, voters will head to the polls to elect the remaining judicial seats.

Lopez Obrador and his successor and mentee President Claudia Sheinbaum argue the judicial overhaul is needed to properly serve the Mexican people and not organized crime.

"Democratizing the judiciary should not be stigmatized," said Morena senator Margarita Valdez. "It is new, but we should not be frightened or afraid."

Critics, however, say the reform does not target those who they consider to be behind the high rate of impunity and corruption in the Latin American nation, such as prosecutors, police and investigators.

The reform has also garnered concern from investors and Mexico's top trade partners, who fear it could weaken the nation's checks and balances and hurt the business climate in the region's No. 2 economy.

"(The legislation passed on Thursday) only serves to concentrate power into a bona fide autocracy," said opposition Senator Susana Zatarain.

"Absolutely nothing about this reform improves access to justice for the average citizen."

Mexico's Supreme Court earlier this month ruled that it would hear a challenge to the reform.

(Reporting by Diego Ore; Editing by Brendan O'Boyle and Lincoln Feast.)