Brazilian presidential candidate Eduardo Campos has died after the small plane that was carrying him and several campaign officials crashed, reports say.
The plane plunged into a residential neighbourhood in the port city of Santos, a City Hall official there said.
All seven people aboard the plane - including a campaign photographer and cameraman, a press advisor and two pilots - died in the crash, City Hall press officer Patricia Fagueiro said.
In a statement on her official blog, President Dilma Rousseff declared three days of official mourning in honour of Mr Campos and said she would suspend her campaign during that time.
"All of Brazil is in mourning," the statement said. "We have lost a great Brazilian."
The 49-year-old was married and the father of five children. The heir of a political family, he served two terms as governor of the northeastern Pernambuco state.
It was not immediately clear whether anyone on the ground was injured in the crash, which took place at around 10am local time. The plane reportedly took off from Rio de Janeiro and was heading to the city of Guaruja.
The Brazilian Air Force reported the plane was a Cessna 560XL. Aeronautical authorities said the craft was attempting to land in bad weather.
Television broadcasts showed wreckage smouldering among smashed buildings in a residential area and emergency response officials picking through the wreckage. Local media reported that a gym was damaged.
A press officer for Mr Campos' running mate, Marina Silva, said she is heading to Santos and media reports said other top politicians were also on their way.
Brazilian vice president Michel Temer called the accident a "tragedy for Brazilian politics".
"Eduardo Campos was a politician of principles and values," he wrote on his website. "Along with the entire country, I am shocked by this accident and by the loss for friends and family."
Eliseu Gabriel, a Sao Paulo city councilman who heads Mr Campos' Brazilian Socialist Party, or PSB, said the party has yet to make any decisions on how to move forward, saying only 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, Mr Gabriel said.
"Eduardo Campos represented a great hope for a profound change in Brazilian politics."
Opinion polls showed Mr Campos running in third place, trailing Ms Rousseff and another candidate in October elections.
His alliance with Ms Silva, a popular politician who previously served as Environment Minister, was seen as giving as boost to his candidacy, although a recent poll showed the ticket trailing with 8%, compared with 36% for Ms Rousseff.