Seven confirmed dead on private jet crashing in bad weather in Santos, south of São Paulo, leaving October election in disarray

Brazil's presidential election campaign was thrown into uncertainty on Wednesday when a private jet carrying the socialist party candidate, Eduardo Campos, crashed into a residential area near São Paulo.

Campos and the six other crew and passengers were killed in the accident, which occurred in bad weather as the Cessna plane was preparing to land.

The deaths prompted a wave of mourning across the country, which is likely to be followed by speculation about the effect on the presidential vote on 5 October.

The president, Dilma Rousseff, declared three days of official mourning for Campos, who she served alongside in the Lula government, who she had served alongside.

"Today we lost a great Brazilian, Eduardo Campos. We lost a great companion … from a young age he fought the good fight in politics as a congressman, minister and two-time governor of Pernambuco," she said. "My condolences to the families of all the victims of this tragedy."

Eduardo Campos attends a campaign event in Rio de Janeiro, Brazil on 13 Aug 2014 Photograph: ZUMA/Rex

Campos, a former Pernambuco governor with a business-friendly reputation, had shaken the political world by choosing the environmentalist Marina Silva as his running mate. The unlikely couple were in third place in the closely fought race to run Latin America's most populous and economically powerful nation. Silva – who came third with almost 20m votes in the last election in 2010 – is now widely expected to head the campaign, though she has yet to comment.

Silva heard the news as she was recording a TV programme and immediately left the studio. Her political party, the Sustainability Network, said she was on her way to the crash site and expressed condolences on Twitter. "We are all shocked by the death of Eduardo Campos in the plane crash this morning," it said.

Other members of the campaign were on their way to the site. "We're stunned. It seems that there are no survivors … An irreparable loss," representative Julio Delgado told local media.

The plane came down in an urban area and crashed into a gymnasium. Images from the scene showed smoke rising from a building and crowds with umbrellas watching as firemen entered the site.

The number of casualties has yet to be confirmed. Six other people, including a press manager, journalist and official photographer, were on board the jet. Reporters said they found election material among the wreckage.

Campos is the grandson of Miguel Arraes, who was also governor of Pernambuco and spent 15 years in exile during the military dictatorship. The political blue-blood was also the youngest member of the first administration of the former president, Luiz Inácio Lula da Silva, in which he served as science and technology minister.

The night before the crash he took part in a live interview on Globo TV in Rio de Janeiro that was expected to lift his polling figures.

"Alongside Marina Silva, I want to represent your frustration, your dreams and your desires for a better Brazil. We will not give up on Brazil. Here is where we will raise our children. Here is where we have to create a fairer society," he told viewers.

"He was one of the most talented public men of his generation. The whole country will be in mourning," said Vital do Rego, the president of the Joint Parliamentary Committee of Inquiry."

The Brazilian Congress halted deliberations. "This is sad news for all Brazilians," federal deputy Izalci Ferreira told reporters.

Romario, the Brazilian World Cup winning striker who is now a successful politician, lamented what he said was a loss to the nation. "Under Eduardo Campo's leadership Brazil would definitely have had a better future," he said.

• This article was amended on 14 August 2014. An earlier version said that Marina Silva was runner-up to Rousseff in the last election in 2010. She came third.