The London police commissioner says a terror attack in the British capital is inevitable.

London's Metropolitan Police Commissioner Sir John Stevens says Britain's security services are working harder than ever trying to foil possible attacks. Speaking at a news conference, he urged the public to be vigilant and report anything considered suspicious, such as unattended bags on trains.

"It is right to say that the anti-terrorist branch, together with M-I5 [domestic intelligence] is probably the most successful counter-terrorist organization the world has seen in the last 32 years," he said. "However, we face a real and very real threat, in terms of what we are facing today. But it is absolutely essential that everybody continues to go about their daily business."

Since September 11, 2001, Mr. Stevens says, 520 suspected terror-related arrests have been made. He says half of the suspects have been charged with offenses and about 90 are awaiting trial.

"Take my word for it, we have foiled major events," he said. "Some of those events will be going through the criminal courts; some, in terms of what has been disrupted, which we cannot talk about, have definitely taken place. Believe you me."

Asked about the bombings in Madrid, the police commissioner said his job is to keep one step ahead of the terrorists.

"We look at tactics. We look at the way these things have been operated and we move ahead," he said. "The way to get ahead with terrorism is to play a chess game. You have to take a look at what has taken place and try and analyze what they are going to do next. And the tactics used, if it was al-Qaida in Madrid, are tactics that they might have used elsewhere. The fact that suicide bombers were not used, and mobile phones, which has been identified, were used as a timing device, is not new."

British police said Monday they were examining suspicious packages sent to four diplomatic missions in London. The Saudi Arabian Embassy has confirmed it received one that contained an unknown white powder.

Scotland Yard later said initial tests indicated that the contents were not hazardous, but additional tests were being carried out.