Speaking at a UN climate change summit, Thunberg said the next decade would define the planet's future, the BBC reported on Wednesday.
She accused world leaders of making constant attempts to find loopholes to avoid making substantial changes.
The teenage activist's appearance came after Brazilian President Jair Bolsonaro called her a "brat".
"The real danger is when politic and CEOs are making it look like real action is happening when, in fact, almost nothing is being done apart from clever accounting and creative PR," the 16-year-old Swede told the COP25 Climate Conference in Madrid, drawing applause.
Summits on climate change seemed "to have turned into some kind of opportunity for countries to negotiate loopholes and to avoid raising their ambition", she added.
The clock was ticking as the decade comes to a close, she said.
"In just three weeks we will enter a new decade, a decade that will define our future. Right now, we are desperate for any sign of hope," she said.
This was meant to be a big moment in the talks, the elixir of the "Greta effect" bringing new energy to a flagging process.
The teenager is almost certainly the most famous person here, attracting far more attention than other celebrities like Al Gore, and the UN badly needs a boost.
Her talk came over as measured, grounded in the latest research, and avoided the flash of hurt and anger she displayed in New York in September.
Looking around the hall, it was striking how many of the national delegations had not turned up for this morning session at the conference.
A snub by the big fossil fuel economies? Or maybe they were too busy in the negotiations themselves?
In any event, the passion among the millions of young people who've taken to the streets to demand action on climate change feels very remote from the diplomatic struggles in these halls.
Thunberg's speech comes after the far-right Brazilian leader lashed out at her after she expressed concern about the killing of indigenous Brazil in the Amazon.
"Greta said that the Ind died because they were defending the Amazon," Bolsonaro told reporters.
"It's impressive that the press is giving space to a brat like that," he added, using the Portuguese word for brat "pirralha".
The activist responded by changing her Twitter bio to Pirralha.
( With inputs from IANS )