The blaze engulfed pavilions in the summit site in Belem around 2:00 pm (1700 GMT), torching a hole through its fabric ceiling, sparking a rush for the exit as smoke filled the corridors and people cried "fire!"

It was the third major incident since the COP30 negotiations started last week in the Amazon region, after Indigenous protesters stormed the venue and later blockaded the entrance in peaceful demonstration.

Security responded "swiftly" and the blaze was controlled in roughly six minutes, COP30 hosts Brazil and United Nations Climate Change said in a joint statement.

Firefighters and ambulances arrived with sirens blaring as acrid smoke billowed from the huge tents and permanent structures hosting tens of thousands of diplomats, journalists, and activists for the two-week event.

Nineteen people were treated for smoke inhalation and two for anxiety, the Brazilian presidency of the event said.

Brazil later declared the site reopened at 8:40 pm after a fire department inspection, with an AFP reporter seeing a trickle of delegates re-entering and food kiosks resuming work -- though plenary sessions will not restart until Friday.

The area where the blaze broke out was sealed off from view with a vast sheet. People who had left belongings in the affected zone were escorted in one by one to retrieve them -- including those who needed to collect passports for flights departing later in the night.

- Shock and disbelief -

Kimberly Humphrey, an emergency medicine physician attending COP30 with Doctors for the Environment Australia, was in a meeting when she received alerts of a fire.

Outside she could "smell smoke and burning plastic. There was no alarms, there was no whistles... only people running and yelling and evacuating."

She found a medical center where people were being treated for shock and smoke inhalation, and started working with others to help those in need.

"It's not what you expect to happen when you are at a conference," she told AFP.

"Initially, there's a sense of disbelief... The first thing I thought was, 'Oh, this isn't real,'" Humphrey said.

"It's really a combination of terror and not having a good emergency plan, not knowing where the exits are, but also what I need to do as a doctor and needing to help people, too."

The cause of the blaze was being investigated but could have been the result of a short circuit or other electrical malfunction, said Brazilian Tourism Minister Celso Sabino.

"This could have happened anywhere in the world," Sabino told reporters.

Early in the summit, UN climate chief Simon Stiell had complained to Brazil about security, sweltering conditions and faulty air-conditioning, and water leaks near light fittings during torrential tropical downpours.

- Bad timing -

Two women who worked in a pavilion belonging to an international organization told AFP the facilities had been fitted with makeshift electrical wiring.

There were exposed wires and water dripped from the roof onto their electrical panel, they said on condition of anonymity, adding that they reported the issues but to no avail.

"The operational difficulties just before the start of this COP could have led people to believe this type of incident might occur," a source close to the organization of the event said, also on condition of anonymity.

The interruption comes at a crucial moment on the second-to-last day of the summit as ministers try to break a deadlock over fossil fuels, climate finance and trade measures.

"It will absolutely delay the process because this is like the crucial time, this the time when we have to decide on the process that started last week," said Windyo Laksono, a member of the Indonesian delegation.

The talks are stuck over a "roadmap" to transition away from fossil fuels proposed by host Brazil, concerns over weak emissions-reduction plans, finance for developing countries, and trade barriers.

Earlier Thursday, UN Secretary-General Antonio Guterres told negotiators "the world is watching" and urged them to reach an "ambitious compromise."