Thousands rallied Sunday in Malaysia in the biggest protest yet against an Australian miner's rare earths plant, as the opposition vowed to shut down the facility if it came to power.

Australia's Lynas has almost completed building the plant near the seaside town of Kuantan in eastern Pahang state to process rare earth ores imported from Australia.

China currently supplies about 95 percent of the world's demand for rare earths, which are used in high-tech equipment from iPods to missiles and have seen prices soar in recent years.

Lynas hopes to begin operations within months, producing an initial 11,000 tonnes of rare earths a year and effectively breaking the Chinese stranglehold on the materials.

But more than 5,000 people, many wearing green and holding banners reading "Stop pollution, stop corruption, stop Lynas," gathered in Kuantan to call for the plant to shut down.

They fear that radioactive waste it produces can seep into the ground and water, harming the environment and people's health.

Lynas has insisted the plant is safe, and any radioactive waste it produces will only be low-level and not harmful to human health.

But opposition leader Anwar Ibrahim, who attended the rally, told protesters his coalition, which currently holds more than a third of parliament, would cancel the plant if it came to power in the next polls expected soon.

"We don't want (this project) to sacrifice our culture and the safety of the children just because of a few corrupted leaders," he was quoted by online news portal Malaysiakini as saying.

His aide confirmed the comments.

Wong Teck, chairman of Himpunan Hijau 2.0, or Green Gathering, a coalition of NGOs that organised the rally, vowed more protests would follow if the government did not shut the plant immediately.

"From the rally today it is clear: We want Lynas out of here," he told AFP. "We are going to go all out all over the country. It is not going to stop here."

Police often intervene at rallies in Malaysia but did not do so on this occasion, although they were on standby.

Lynas' website was also hacked with a Malaysian flag and the slogan "Stop Lynas, Save Malaysia" replacing the usual site.

A Lynas representative could not immediately be reached for reaction. A Lynas advertorial in the New Straits Times daily Sunday said the company met "rigorous health, safety and environmental regulations."

Lynas is receiving a 10-year tax break for the plant, and has said having it in Malaysia offers better economics than Australia.

The government has said it is monitoring the plant closely to ensure its safety. Prime Minister Najib Razak was quoted by The Star daily as saying the plant is scientifically save, accusing the opposition of playing up the issue.

On Tuesday the Kuala Lumpur high court will start hearing an activist challenge to block the plant.

Opponents point to a similar rare earths plant in Malaysia's northern Perak state which was forced to shut down in 1992 over protests from residents who blamed it for birth defects in nearby populations.