SANTOS, Brazil -- Brazilian presidential candidate Eduardo Campos died Wednesday when the small plane that was carrying him and several campaign officials plunged into a residential neighbourhood in the port city of Santos.

All seven people aboard the plane, including a campaign photographer and cameraman, a press adviser and two pilots, died in the crash, Santos City Hall press officer Patricia Fagueiro told The Associated Press.

In a solemn address, President Dilma Rousseff declared three days of official mourning in honour of Campos and said she would suspend her campaign duringthat time.

"Today Brazil is in mourning and reeling from a death that took the life of a promising young politician," she said, adding that Campos had been facing "an extremely promising future."

Campos, the scion of a political family from the northeastern state of Pernambuco, had been an ally of Rousseff but broke away ahead of the campaign for the Oct. 4 presidential election.

Polls suggested he was running in third place, far behind Rousseff and another political rival. But his Brazilian Socialist Party ticket was widely regarded as among the best-placed to challenge Rousseff and her Workers Party, thanks largely to his popular running mate, former Environment Minister Marina Silva, who joined Campos' ticket after her attempt to run for president herself failed.

It was not immediately clear whether Silva would assume Campos' spot as the party's presidential candidate. Under Brazilian law, in the event of a candidate's death, a party has 10 days to decide on a substitute.

A visibly shaken Silva spoke to reporters in Santos, reading a prepared statement in which she concentrated on her relationship with Campos and gave her political future a wide berth.

"During these 10 months of partnership, I learned to respect him, admire him and feel confidence in his attitudes and his ideals in life," Silva said in a soft, wavering voice. She rose from the microphone before the press could pepper her with questions.

Pundits were already predicting that Campos' death could complicate the presidential race for Rousseff.

Brazilian television broadcast a continuous loop of images of the wreckage, a smouldering pit between buildings of several stories, with emergency workers picking through the rubble. Brazil's top broadcaster, Globo, ran interviews with eyewitnesses who reported the plane was already ablaze before the crash around 10 a.m.

This video posted to Facebook appears to show a plane crash in the port city of Santos on Wednesday, August 13, 2014.

Aeronautical authorities said the Cessna 560XL was attempting to land in bad weather.

Sao Paulo state security officials said it appeared there were no on-ground deaths. The newspaper Folha de S.Paulo reported five people on the ground were injured.

The plane took off from Rio de Janeiro, where Campos appeared in a television interview Tuesday, and was headed to the city of Guaruja, where he was to participate in a conference about Brazil's ports.

The country's top politicians expressed shock and sorrow over the accident, with Vice-President Michel Temer calling it a "tragedy for Brazilian politics."

"Eduardo Campos was a politician of principles and values," Temer wrote on his website. "Along with the entire country, I am shocked by this accident and by the loss for friends and family."

Former President Luiz Inacio Lula da Silva called Campos "a public figure of rare and extraordinary quality."

"The affection, respect and mutual admiration were always present in our dealings with one another," Silva said in a statement.

The spokeswoman for the Obama administration's National Security Council, Caitlin Hayden, said, "We were deeply saddened to learn of the plane crash."

"The thoughts and prayers of the American people are with Brazil on this tragic occasion," she said.

Eliseu Gabriel, a Sao Paulo city councilman who heads Campos' party in Sao Paulo, said party officials had yet to make any decisions on how to move forward, saying only that the campaign was "stopping" for the moment.

"The campaign was about to start, and he had a big chance of making it to the second round" of Brazil's two-round race, Gabriel said. "Eduardo Campos represented a great hope for a profound change in Brazilian politics."

As the initial shock of the crash passed, pundits began to speculate about how Campos' death would affect the presidential race.

David Fleischer, a political scientist at the University of Brasilia, said Campos' death was "bad news for Brazil and very bad news for Dilma."

Should Campos' running mate assume the candidacy, she would likely pull votes away from Rousseff, forcing the race into a second-round ballot between Rousseff and the other main candidate, Aecio Neves of the Brazilian Social Democracy Party. Silva's support of Neves in a runoff could threaten Rousseff's chances of re-election, Fleischer said.

Rousseff, the hand-picked successor of ex-President Silva, has seen her popularity flag in recent months amid popular dissatisfaction with slowing growth, high taxes and poor public services -- although she has remained the strongest candidate. A survey by the Datafolha polling agency released Wednesday before the accident said 8 per cent of those questioned said they intended to vote for Campos, compared with 36 per cent backing Rousseff.

Campos, 49, was married and the father of five children, the youngest of whom was born in January. The heir of a political dynasty that stretched back to his grandfather, he served two terms as governor of Pernambuco state.

His brother, Antonio Campos, told Globo and other broadcasters that Campos would be buried in the family tomb in Pernambuco, where his grandfather's body lies. The grandfather, Miguel Arraes, died on the same date, Aug. 13, nine years ago.