London anti-terror raids occurred across the country yesterday as anti-terror police arrested four men suspected of planning attacks in the U.K., reports the International Business Times. London anti-terror police suspect that the men are part of Islamic terrorists.

London anti-terror police said the arrests were part of an investigation into Islamist terrorism. The men were reportedly in their early 20s.

Though armed police were present at one of the London anti-terror raids, no shots were fired as the search warrant was executed, said London's Metropolitan Police.

On Tuesday, police said officers used a stun gun at one of the suspects, a 21-year-old male who did not require medical treatment after the London anti-terror raid.

According to Scotland Yard, armed police from specialist firearms unit SCO19 attended one of the London anti-terror arrests after surveillance showed suggested it was possible for the suspect to have access to weapons.

Police said the group of young men have been arrested on suspicion of preparing or instigating acts of terrorism, reports CBS and the Associated Press.

"These arrests and searches are part of an ongoing investigation into Islamist related terrorism," according to a statement. "A number of residential addresses and vehicles are being searched by specialist officers in west and central London as part of the investigation."

Whitehall officials said they believe the men arrested during the London anti-terror raids were undergoing the early stages of planning a terrorist attack, though no explosives were discovered. The men reportedly could have been monitored for some time by London anti-terror investigators and the arrests occurred after they have gathered enough material against the men.

The suspects are still in custody. Meanwhile, homes and vehicles in west and central London are continued being searched as police are carrying out so-called ongoing investigation into Islamic-related terrorism.

London anti-terror experts have reportedly predicted long before that jihadists returning from Syria could possibly commit terrorism in the U.K. Possible attacks in the U.K. are now feared more so than ever as Britain recently joined the Coalition and carried out air strikes against Isis in Iraq.

The AP reports that terror threat in the U.K. was raised to "severe" recently as Britons traveling to Syria to join up with the Islamic State of Iraq and Syria have increased.

The raise of the terror level to severe in August meant that attacks are highly likely. The change came after an advice from the Joint Terrorism Analysis Centre. "Critical" is reportedly the highest level is, which meant a terror attack is expected to happen imminently.

According to London anti-terror experts in the government, militant groups are planning to strike Britain.

Current estimates of Sunni extremists from Europe who have traveled to Syria since January 2012 are between 1,500 and 2,500. The count includes groups like the Nusra Front and the Islamic State of Iraq and Syria (ISIS), the militant group that has taken over the northern part of Iraq. The estimates were predicted by Seth Jones, a former U.S. counter-terrorism official.

Last month, British police took part in London anti-terror actions when they arrested 11 people in other Islamist-related operations. It was not immediately clear if the investigations were connected, according to NBC News.

London anti-terror comes after a sharp spike in the number of Western jihadis triggered major concern amongst U.S. and Europe leaders

Social media was also abuzz with the current news.