London police staged a series of overnight raids on Thursday to quash a suspected Remembrance Day terror plot that may have involved an attack on the Queen, according to British reports.

Scotland Yard remained on high alert Saturday after four men were arrested on suspicion of “Islamist-related terrorism,” police said. Authorities suspect the alleged terror plot may have been linked to Remembrance Sunday plans involving Queen Elizabeth II and the Royal Family.

Four men between the ages of 19 and 27 were arrested in the London-area raids. They are being held on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism.

The arrests come amid growing concern in the U.K. that homegrown extremists might return from fighting with the Islamic State to commit acts of terrorism on British soil.

The lone gunman attack on Parliament Hill in Ottawa last month has also triggered renewed fears of a similar anti-military attack in the U.K.

In August, the U.K. raised its terror alert level to the second-highest on its scale, from “substantial” to “severe.” Officials have beefed up security in London ahead of Sunday’s Remembrance Day ceremonies, which typically involve the Queen laying a wreath at the cenotaph.

Britain honours its fallen soldiers each year on the second Sunday of November, called Remembrance Sunday. The Queen and the Royal Family are still scheduled take part in the annual ceremony.

British citizens mark Nov. 11 as Armistice Day, the anniversary of the end of hostilities during the First World War in 1918.