Antananarivo (AFP) - A toddler died in hospital Monday following a grenade attack in Madagascar's capital, bringing the death toll to three in what the president called "an act of terrorism".

The blast struck the Mahamasina municipal stadium in Antananarivo at around 1600 GMT Sunday, just as a free concert was taking place to mark the nation's 56th anniversary of independence from France.

According to the gendarmerie, the attack immediately killed two teenagers aged 16 and 18.

"There are now three dead," including the 14-month-old girl who died of her wounds, Prime Minister Olivier Mahafaly Solonandrasana said Monday, adding that 91 people were injured in the attack and an enquiry was under way.

President Hery Rajaonarimampianina, who visited the wounded in hospital, blamed the attack on tensions with political opponents in the Indian Ocean island nation.

"There may be differences of opinion between us, but these acts of destabilisation are unacceptable," he said in a statement broadcast on national television, describing the attack as "not just a destabilising act but an act of terrorism".

Pleading for calm, he added: "We will not respond to violence with violence."

"I do not believe that a divergence of views pushes people to commit such an atrocity " said former prime minister Omer Beriziky, one of the voices critical of the current regime.

- 'Terrorist act' -

"The explosion was caused by a grenade," general Anthony Rakotoarison, head of security and intelligence with the national gendarmerie, told AFP by phone.

"We consider this a terrorist act," he added.

A military parade had been held at the stadium earlier Sunday.

One of those injured, 15-year-old John Joelison, said there were three security checks at the stadium.

"So I can't understand how the attacker managed to get the bomb in," he said.

However a medical source said that the security forces had rapidly been overwhelmed, letting people come and go without being searched.

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The last attack to hit Madagascar was in January 2014 when a grenade blast killed a toddler and injured several other people outside the same stadium targeted on Sunday.

No arrests were ever made in connection with that attack and there was no claim of responsibility.

Madagascar, one of the world's poorest countries, is slowly getting back on its feet after a lengthy period of political instability triggered by the 2009 ouster of president Marc Ravalomanana by Antananarivo's then-mayor Andry Rajoelina.

Rajoelina led a transitional government until late 2013, when a new election that was designed to resolve complex struggles brought Rajaonarimampianina to power.

International donors, on which the country relies heavily, only recently returned to Madagascar after withdrawing over the 2009 turmoil, and the economy is starting to show the first signs of recovery.