image copyrightReutersimage captionAn estimated 93.5 million Mexicans were eligible to vote in the mid-term elections
Mexico's ruling coalition is expected to lose ground in Congress after one of the country's bloodiest campaigns in recent history.
Early results of Sunday's mid-term poll suggest President Andrés Manuel López Obrador's governing Morena party and its allies will win between 265 and 292 of the 500 seats in the lower house.
This is short of the two-thirds needed to approve sweeping changes.
The election was marred by violence with dozens of politicians killed.
On Sunday, a severed head was lobbed into a polling station and five election workers were shot dead.
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The election was seen as a referendum on the leftist rule of 67-year-old Mr López Obrador, who is half-way through a six-year term.
If the results are confirmed, Mr López Obrador is likely to face challenges to the continuation of a programme of reforms he calls the "Fourth Transformation", with an overhaul of politics and the economy.
But his coalition may still gain an absolute majority together with allies.
Some 51% of an estimated 93.5 million eligible voters came out to vote.
All 500 seats in the lower house, 15 state governorships and thousands of local leadership positions are up for grabs.
image copyrightEPAimage captionPresident Andrés Manuel López Obrador was hoping to keep his party's two-thirds majority
Despite maintaining high approval ratings, Mr López Obrador, 67, has faced increasing criticism for his handling of the Covid-19 pandemic, and his failure to crackdown on drug cartels.
Mexico's main opposition parties have formed an electoral and legislative alliance, and are seeking to overturn the government's dominance in the legislature.
Violence continued on polling day
Five election workers were shot dead in the state of Chiapas. In Mexico state, someone lobbed an inactive grenade into a voting station.
In the border city of Tijuana, a man threw a severed head at a polling station and plastic bags filled with body parts were found nearby, local authorities said.
Elsewhere, videos showed activists attacking polling stations and voting had to be suspended in some places.
media captionWatch: the Mexican mayor refusing to give in to violence as she stands for a second term