British Prime Minister David Cameron was preparing Thursday for a key vote on joining airstrikes in Iraq as police arrested nine people, including a notorious radical preacher, accused of links to Islamist extremism.

Cameron was meeting ministers after returning from the UN General Assembly in New York, where he announced that parliament would vote Friday on British planes joining US-led strikes against the Islamic State (IS) group in Iraq.

With the prime minister set to argue that the jihadists pose a direct threat to Britain, nine people were arrested in London early Thursday on suspicion of encouraging terrorism and belonging to and supporting a banned organization. […]