Venezuela's former vice-president Nicolas Maduro has been sworn in as the interim president in the capital Caracas following the funeral of Hugo Chavez.

The swearing-in ceremony, conducted by Speaker Diosdado Cabello, took place in the National Assembly as fireworks lit the sky.

"I swear by the most absolute loyalty to comrade Hugo Chavez that we will fulfil and see that it's fulfilled the constitution ... with the iron fist of a people ready to be free," said Maduro, the preferred successor of Chavez.

Maduro has appointed Chavez's son-in-law Jorge Arreaza as vice-president. The date for the presidential elections is expected to be announced later in the day.

The ceremony was boycotted by the main opposition. The opposition insists that Cabello should take over as acting president under the constitutional authority and claims the swearing-in is "fraudulent."

"Nicolas, nobody elected you president. The people didn't vote for you, kid," said the opposition leader and the expected presidential candidate, Henrique Capriles.

Soon after the ceremony, Maduro urged the electoral council to call for an election "immediately."

Maduro also took the opportunity to touch upon suspicions surrounding Chavez's death that it was instigated by his adversary states.

Maduro said: "This illness very strange for the speed of its growth and for other scientific reasons that will be known in their moment. We tell them: Sooner than later, the imperialist elites who govern the United States will have to learn to live with absolute respect with the insurrectional people of the ... Latin and Caribbean America."

Chavez's funeral was attended by more than 30 leaders of states including Iranian President Mahmoud Ahmadinejad and Alexander Lukashenko of Belarus.

"We have lost a great leader, a great man. Hugo came from the people and he served the people," said Ahmadinejad after the ceremony.

"He was invincible. He left victorious and no one can take that away. It is fixed in history," said Cuban President Raul Castro.

Syrian President Bashar al-Assad's message was read out during the funeral.