Belgian police arrested several people planning a terrorist attack on European Commission Headquarters in Brussels, media reports say. It comes amid fierce efforts by local authorities to crack down on extremism.

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Two of those arrested were from the The Hague, Dutch public broadcaster NOS reported.

“They were planning an attack. One of the targets was the European Commission building. Individual commissioners were not particularly targeted. It would be more like the Jewish museum attack… with the aim to kill as many people as possible,” NOS reported citing an informed source.

Belgian police reportedly found guns and bulletproof vests, while Dutch police uncovered jihadist literature.

The arrests and seizure of arms resulted from simultaneous home raids in Brussels and The Hague. One raid was reportedly carried out in The Hague’s Schilderswijk district, where a demonstration in support of the Islamic State (IS, formerly ISIS / ISIL) recently took place.

NOS reported that a man and woman of Turkish descent were arrested early last month when they arrived in Brussels on a flight from Turkey. Both had allegedly been in Syria.

The arrests come as Belgium’s authorities are investigating extremism in the region and have detained a number of people suspected of having links with jihadists, Belgian daily L’Echo has said.

In May, four people were killed in a gun attack on the Jewish museum in central Brussels. The suspect in the attack, Frenchman Mehdi Nemmouche, is believed to have spent over a year fighting for IS militants in Syria.

Reportedly, there are 400 Belgians fighting in Syria on the side of the extremists and around 90 of them have returned home.

“Our starting point is that among them, one out of nine aim[s] to carry out an attack,” L’Echo’s source said. “That is a conservative estimate, if you also take into account the people who help them.”

“We are working full time on the problem of the returning fighters,” Belga news agency cited a federal judicial spokesman as saying. “We work together with the security services and that led us to carry out several operations,” the spokesman said.

Belgian efforts come as the rest of Europe, as well as the US and Australia have carried out mass arrests amid frightening intelligence reports of IS terrorist threats.

READ MORE: IS-related arrests in US, Europe, Australia as intel warns of ‘gruesome’ attacks

Many Western countries worry that their involvement in fighting for extremists in the Middle East will strike back with attacks on home soil.

In Germany, up to nine citizens carried out suicide bombings in the name of the Islamic State this year, a joint German media research team reported.

Meanwhile, Australia has raised its terror alert to high for the first time in 11 years. Following serious allegations by the officials that an attack is in fact being planned, the authorities then detained 15 people on suspicion of planning a random attack in Australia.