Mexico's opposition centrist Institutional Revolutionary Party (PRI) improved its standing in legislative elections against conservative President Felipe Calderon's party, exit polls showed.

The ruling National Action Party (PAN) stands to lose some of its 206 seats in the Chamber of Deputies, where it is the strongest party, despites hopes that its crackdown on drug cartels across the country would reinforce its standing.

Polls closed at 6:00 pm (2300 GMT) in most parts of the country, while voting ended at 0100 GMT in the northwestern states for the midterm elections, which were dominated by the explosion of violence linked to cocaine trafficking and the economic recession.

Worries about the violence were partially eclipsed by concerns over swine flu, which first emerged at the end of April in Mexico.

Some 77 million Mexicans are eligible to vote in the elections to choose who will hold 500 seats in the Chamber of Deputies, six governorships and 568 mayoralties around the country but early exit polling showed a high abstention rate.

In a grim reminder that drug-linked violence has not abated, six men were killed overnight Saturday in the northern state of Chihuahua, which saw 325 deaths from organized crime just last month.

Four bodies were found in the state's capital, also called Chihuahua, including two in the trunk of a car.

Another body was riddled with at least 16 large-caliber bullet holes and found in a car registered just across the border in the US state of Texas.

The elections proceeded without major incidents, though on Saturday a PRI gubernatorial candidate in San Luis Potosi state said his vehicle had been shot at, allegedly by militants.

Meanwhile, PAN leaders said a molotov cocktail was thrown Sunday at a house where some of its supporters were gathered in Toluca, capital of central Mexico state, but no casualties were reported.

Bogged down by fierce internal conflict, the leftist Democratic Revolutionary Party is likely to slip from second strongest party to third.

That would give the PRI, which held government for much of the last century, a chance to regain a hold on the electorate.

PAN, a right-leaning Catholic party, has run on the president's high-profile security initiatives, but the country's disastrous economic outlook is expected to prove decisive in swaying voters.

Mexico is facing its worst economic crisis in 15 years, with its economy shrinking 8.2 percent in the first quarter and over half of the population living below the poverty line.

In the midst of the dim outlook, PRI has sought to return to power following its historic 2000 drubbing by PAN, which cut short what had been a 71-year run in power.

Despite its third-place status in parliament, it has retained a lot of influence in Mexico's poorest states.

The outbreak of the A(H1N1) virus led to a virtual shutdown of the country and scared off tourists. It has since been declared a global pandemic, killing at least 382 people and infecting tens of thousands worldwide.

Drug violence involving a vicious circle of murder, recrimination, inter-gang wars and confrontation with the Mexican army has left some 10,000 people dead since late 2006.

Calderon has dispatched around 36,000 troops across the country, as part of a muscular campaign to clamp down on drug cartels.

Despite the spike in violence, the government's strategy against the cartels enjoys the support of 80 percent of Mexicans, according to Mitofsky, a Mexican consulting group.