ANKARA - Turkey's deadliest earthquake in a generation has handed President Tayyip Erdogan a huge rescue and reconstruction challenge which will overshadow the run-up to May elections already set to be the toughest of his two decades in power.

A day after the quake struck, killing more than 3,500 people in Turkey, opposition parties and some residents in worst-hit areas complained that authorities were slow or ill-equipped to react to the devastation.

Any perception that the government is failing to address the disaster properly, or had not enforced adequate building codes in a country prone to earthquakes, could hurt Erdogan's prospects in the vote.

But analysts say the president, a skilled campaigner whose government has tackled earthquakes, wildfires and other natural disasters since he came to power in 2003, could rally national support around the crisis response and strengthen his position.

Speaking just hours after Monday's quake, which he described as the worst to hit Turkey in more than 80 years, Erdogan said thousands of rescue workers had already mobilised and no effort would be spared in the harsh winter conditions.

The government declared a "level 4 alarm", calling for international assistance, and a three-month state of emergency in the most affected provinces.

"Erdogan responded rapidly and coherently to the crisis," the Eurasia Group consultancy said. "That is likely to burnish his strong leader image ahead of 14 May elections — if the government can maintain its early momentum."

'BLACK SWAN'

Reconstruction costs are likely to run to many billions of dollars, straining an economy already hit by 58% inflation. The disruption in a region that is home to 13 million people is expected to curb growth this year, economists say.

The scale of the damage, across hundreds of kilometres and affecting millions of people and their homes, would "completely reset" Turkey's economy and politics, said Atilla Yesilada of Global Source Partners.

Describing the 7.8 magnitude quake as a "black swan", an occurrence so unforeseen or unlikely that it could have extreme consequences, he said it was not yet clear whether elections could even be held in the hardest-hit regions.

Erdogan's political opponents have not rushed to make political capital in the immediate aftermath of the quake, as people remain trapped under buildings and the death toll rises.

The six-party opposition said only that the government should work "without discrimination" to address the disaster that hit regions including Kurdish communities and Syrian refugees.

But Ugur Poyraz, Secretary General of centre-right nationalist IYI Party, said he had toured severely hit areas and as of Tuesday morning seen no sign of emergency rescue workers.

"There is definitely no professional aid coordination," he told Reuters. "Citizens and local teams are joining the rescue operations by themselves to save people in the rubble."

Authorities say more than 12,000 search and rescue personnel and another 9,000 troops are in action.

In a close-run election, the government's response to the emergency could sway crucial middle ground voters, although it is unlikely to sway committed supporters of either side, said Hasnain Malik, managing director of emerging and frontier markets equity strategy at Tellimer in Dubai.

"The response of Erdogan's government to this natural disaster might shape the attitude of the floating voter but the loyalties of most voters are already determined."

(Additional reporting by Bansari Mayur Kamdar in Bengaluru; Writing by Dominic Evans; Editing by Jonathan Spicer and Nick Macfie)