Marina Silva is what the enlightened future was supposed to look like. Smart, determined, effective and focused on indigenous culture, education and environmental conservation, the former minister and presidential candidate was once a symbol of hope for what Brazil and the world could be.

But despite her prominent global profile – evidenced by the fact that she was the only Brazilian to carry the Olympic flag during London's opening ceremony – Silva has recently fallen into the shadows domestically, and the sustainability agenda she champions has been sidelined by a government that is using the global economic crisis to push forward with mining, monoculture and infrastructure investment.

These are tough times to be an environmental campaigner, but Silva has responded with a characteristically groundbreaking move. Earlier this year, she formed a political party that aims to channel the power of social networks against big corporate interests obsessed with growth at all costs.

It has collected more than 100,000 signatures in its first month, a considerable step towards the 560,000 needed to put forward a candidate in next year's presidential election.

Silva secured 20m votes at the last poll, in 2010 – the largest vote for any Green candidate anywhere in the world – but lost to the current president, Dilma Rousseff.

Silva has not yet formally declared her intention to run again in 2014, but in an interview with the Guardian she said she felt it was essential to stand against a government that she believes has lost its way.

"I don't know whether it will be harder this time, but I know it will be more important because many people now have given up hope," she said during a visit to a school in Rio de Janeiro. "We live in a period of grave crisis – economic, social, environmental, political and ethical. It's a crisis of our civilisation. It's not easy for people to face up to something like that. The effort to shift the world in a better direction has lost momentum."

The creation of the party, Sustainability Network, is the latest move in a remarkable career. One of 11 children in a family of rubber-tappers from the impoverished north of Brazil, Silva worked as a maid, then entered university and became a student and union activist. She was an associate of Chico Mendes, the environmental activist murdered for his campaign to protect the Amazon, and prominent in the Workers' party, which won power for the first time in 2002 under President Luiz Inácio Lula da Silva.

Marina Silva was among the flag-bearers at the London 2012 Olympics opening ceremony. Photograph: Chamussy/Niviere/Sipa/Rex Features

As environment minister, she won plaudits for introducing the most effective strategy against deforestation the country has ever seen, for changing the Brazil's policy on climate change, for creating extensive new protected areas, and for the establishment of the Amazonian fund.

But she quit the government in 2008 when the agriculture ministry and big-cattle and soy-growing regions such as Mato Grosso pressed the president to relax restrictions on land use.

"I left because of the big tensions in the government over deforestation measures," she said. "Our policies had started to produce good results and there was a strategic move to dismantle them."

Her case has parallels with other South American nations where left-leaning governments came to power with promises to protect the environment and respect the rights of indigenous people that proved difficult to keep. In Ecuador, Alberto Acosta – who headed the panel that rewrote the country's "green constitution" – recently stood against his former ideological ally, Rafael Correa, because he believed the president had abandoned his principles.

Whether governments have changed course because of the economic crisis or cynical realpolitik is uncertain, said Silva.

"I don't know if they were ever genuine about the environment or whether they just used it for electoral convenience. Lula really made a difference in society. But the old left has difficulty accepting new ideas. The revolutionaries of our era are the conservationists."

Under Lula's successor, Rousseff, she believes the rollback has been more pronounced, particularly since the revision of the forest code last year.

Such criticism and the likelihood that Silva will run again for president in 2014 meant many Brazilian eyebrows were raised when she was chosen to carry the Olympic flag in London. Some commentators thought this was an intrusion into domestic politics.

Silva believes they missed the point.

"It was immensely gratifying to see the British have the sensibility to make the sustainability issue one of the great causes of humanity," she said. "I also felt immense responsibility to be the representative of Brazil, which is the country best placed to promote these issues and break with the old ideas. We must not let the economic crisis affect the environment. We must invest in the new economy. There is no time to waste."

Not everyone is convinced. Silva remains popular, but her religious zeal – she has switched from Catholicism to evangelical Christianity – has turned off some potential supporters.

Her message mixes environmental advocacy with an appeal for greater use of the internet to engage voters who previously felt distant or disenfranchised from the political process.

"The new party recognises that people no longer want to be mere spectators. They want to be actors," she said. "We want to show how to transform reality in the direction of sustainable development. Brazil is the best place for this … The people recognise that, but not the government."

However, in these times of economic concern, it is unclear whether the public will respond to her as they did last time. Polls suggest Silva is a long way behind Rousseff, who has adopted many of her rival's proposals on education.

Declaring determination as more important than optimism or pessimism, Silva said her past campaigns had shown how surprises can be achieved.

"People don't understand how I got 20m votes. I say we were like a samba school. Everyone was preparing separately in their own homes, but they were practising the same song and everyone knew their roles. So when it was carnival time, it all worked beautifully."

A new party has begun. But half a million signatures must be collected before the new party can re-enter the presidential parade. The future offered by Marina Silva continues to look enlightened, but – for the moment, at least – as distant as ever.

Additional reporting by Marcela Bial