Several Mexican cities have seen large-scale protests against what the opposition claims are attempts by the government to undermine the integrity of the electoral process.
The largest was in Mexico City, where 500,000 people, according to the organizers, marched through the center of the city. The number was 90,000 according to the local government.
Last week, lawmakers approved cutting the National Electoral Institute's (INE) budget and staffing.
The INE is branded partisan by President Andrés Manuel López Obrador.
Opponents, however, have called for the Supreme Court to overturn the recent vote as unconstitutional, describing it as an assault on democracy itself.
The historic Zocalo sq\. in Mexico City saw large crowds on Sunday. In the heart of the city, the protesters overflowed into neighboring streets.
Protester Veronica Echevarria was quoted by Reuters as saying, "We're fighting to defend our democracy.". She was donning a cap bearing the slogan "Hands off the INE.". Similar-sounding cards were carried by numerous protesters.
In several other cities, smaller, peaceful protests were held.
The reforms were approved by the Mexican Senate on Wednesday, following a similar decision by the lower house of the legislature. Following President López Obrador's signature, the reforms will take effect.
It may be the most divisive political topic in Mexico right now, according to Will Grant, the BBC's correspondent there.
After two unsuccessful elections, Mr. López Obrador was chosen in July 2018. He has long been critical of the INE, whose employees supervise elections.
In a previous statement, he accused the independent body of fraud and claimed that its employees covered up "the stuffing of ballot boxes, falsifying of [election] records, and vote buying.".
In his first run for the presidency, in 2006, he fell just short to his conservative opponent Felipe Calderón. For months, Mr. López Obrador steadfastly refused to accept the outcome, which he branded fraudulent.
In the 2012 election, he ran against Enrique Pea Nieto and challenged the outcome.
Since his victory in 2018, Mr. López Obrador has been advocating for the INE's reform, which he claims will save taxpayers $150 million (£125 million) a year by significantly reducing the agency's workforce.