Mexicans head to the polls Sunday in legislative elections that amount to a midterm test for conservative President Felipe Calderon as the country remains mired in brutal drug-fueled violence.

Some 77 million Mexicans are eligible to vote in the elections to choose who will occupy 500 seats in the Chamber of Deputies, six governorships and 568 mayoralties around the country.

Voting was scheduled to begin at 1300 GMT and end at 2300 GMT.

The explosion of violence linked to cocaine trafficking and the economic recession dominated the electoral campaign, which was partially eclipsed by worries over the swine flu that first emerged at the end of April in Mexico.

The A(H1N1) flu virus, which led to a virtual shutdown of the country and scared off tourists, has since been declared a pandemic, killing hundreds of people and infecting tens of thousands worldwide.

The vicious circle of murder, recrimination, inter-gang wars and confrontation with the Mexican army has left some 10,000 people dead since late 2006, despite Calderon dispatched some 36,000 troops across the country as part of a muscular campaign to clamp down on the cartels.

Calderon's ruling National Action Party (PAN) could lose its slim majority in parliament, with most polling finding that the third party opposition, the centrist Institutional Revolutionary Party (PRI), is set 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.

Some politicians were also targets of violence.

Less than 24 hours before voting began, a mayor in the southeastern town of Siltepec, one of the poorest municipalities in Chiapas state, received a hand grenade wrapped in gift box.

The package also contained a cellular phone and an unsigned letter that read "Let's see if you will now answer the calls," a local police official told AFP on condition of anonymity.

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

In the midst of the dim outlook, PRI has sought to return to power following its historic 2000 drubbing by PAN that cut short what had been a 71-year run in power. Despite its third-place status in parliament, it has retained much influence in the poorest states.

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

At the close of the campaign, PRI enjoyed a 5-percent lead over PAN's 33 percent, according to a poll published by the Reforma newspaper on Wednesday.

"I do not think this has been a very successful campaign for PAN. People must not be very happy with Calderon's government because that is what is reflected here," Mexican writer and political analyst Jose Antonio Crespo told AFP.

"They played the security policy card because they thought that was the best one they had. But it seems that it will not be enough."

Less than half of eligible voters -- about 30 million -- were estimated to make it to the polls.

The high political apathy reflected substantial skepticism both of political elites and of politicians' ability to provide physical and economic security to the 47 percent of the 103-million strong population that subsist in poverty.

Despite the tough political climate some celebrities nonetheless sought to make a run for power.

Among the public figures were former sprinter Ana Guevara, Laura Esquivel -- writer of the best-selling novel "Like Water for Chocolate" -- and Lucia Morrett, who survived an attack against Marxist Revolutionary Armed Forces of Colombia (FARC) rebels in Ecuador last year.