Authorities in Paris have sealed off the streets surrounding fire-damaged Notre Dame Cathedral to allow the area to be decontaminated after the site around the landmark tested positive for high levels of lead following April's devastating blaze.

Soldiers and police officers cleared the streets and a bridge around the monument as high fences were erected to keep people out of the new decontamination zone.

Several schools lie within the zone and activists and residents have accused the authorities of underestimating the threat of lead poisoning.

Workers direct a mechanical shovel grabbing pieces of destroyed surfacing to gather up the lead particles. Credit: AP

Experts are using a mixture of decontamination techniques. One uses high pressure water jets with chemical agents, while another involves spreading a gel on public benches, street lights and other fixtures to absorb the lead, letting it dry for several days before removing it.

Last week, workers began decontaminating some schools tested with unsafe levels of lead and city authorities say all schools are expected to be decontaminated when children return to school in early September.

The operation is expected to take about three weeks, although police union official Frederic Guillo said there was a chance it could be extended to neighbouring streets.

Notre Dame was in the initial stages of a lengthy restoration when fire broke out in the evening of April 15 and swiftly took hold, threatening to destroy the 850-year-old Gothic cathedral.

Rubble surrounding the altar following the devastating fire in April. Credit: AP

Several tonnes of lead melted in the inferno that consumed the iconic spire and destroyed the roof. But a massive operation by French firefighters saved Notre Dame’s famous bell towers, rose windows, organ, and precious artworks.

Still very much in its preliminary stages, the painstaking cleanup work inside Notre Dame was suspended last month for safety reasons.

Authorities put up fences to keep people out of the decontamination zone. Credit: AP

Work is continuing on the new decontamination zone for workers who have been clearing hazardous debris from inside Notre Dame, to ensure that their activities don't generate any pollution outside the work zone.

In an address to the nation following the fire that stunned the world, French president Emmanuel Macron vowed to rebuild the cathedral in just five years and launched an international fundraising scheme to fund the project.

In response, at least half a billion euros has been donated by wealthy billionaires and businesses to help rebuild the iconic building.

Many of the artefacts that were saved were sent to the Louvre Museum including the crown of thorns, one of Notre Dame's most precious treasures that the Roman Catholic Church believe was placed on Jesus' head before his crucifixion.