London (CNN) -- The man in charge of security for the upcoming Olympic Games in London said Monday that preparations have been "long and thorough," and expressed confidence that the event will prove to be a success.
"The national threat level at present is assessed to be substantial -- meaning that an attack is a strong possibility -- one notch lower than has been the case for much of the last 10 years," said Jonathan Evans, director general of the Security Service, known as MI5, in an address at Mansion House, the official residence of the mayor of London.
"We are far from complacent. A lot of hard work still lies ahead and there is no such thing as guaranteed security. But I think that we shall see a successful and memorable Games this summer in London."
Evans said members of his service had been involved in advising on the design and security of the venues, in the accreditation of those working there and in intelligence collection and analysis. "The preparations have gone well," he said.
"No doubt some terrorist networks have thought about whether they could pull off an attack," he acknowledged. "But the games are not an easy target and the fact that we have disrupted multiple terrorist plots here and abroad in recent years demonstrates that the UK as a whole is not an easy target for terrorism."
The Royal United Services Institute, a defense and security think tank, has identified 43 potential plots or serious incidents in Britain since the terrorist attacks in the United States on September 11, 2001, he said.
Evans credited an investment in counterterrorism with having succeeded since then in "identifying, disrupting and suppressing terrorism before it succeeds. You could say that we are near to reaching a form of stalemate -- they haven't stopped trying but we have got better at stopping them."
He added that he was seeing a change in international threats, with incidents involving Pakistan and/or Afghanistan falling from about three-quarters of all such threats to fewer than half.
"We appear to be moving from a period of a deep and focused threat to one where the threat is less monolithic but wider. Al-Qaeda affiliates in Yemen, Somalia and the Sahel (region of Africa) have become more dangerous as al Qaeda in Pakistan has declined and we see increasing levels of cooperation between al Qaeda groups in various parts of the world."
In addition, "a small number of British would-be jihadis" have been traveling to Arab countries to find training and opportunities for militant activity, he said. "Some will return to the UK and pose a threat here. This is a new and worrying development and could get worse as events unfold. So we will have to manage the short-term risks if there is to be a longer-term reward from the Arab Spring."
The Security Service, or MI5 is the British domestic spying agency -- sister of MI6 foreign intelligence service, where fictional spy James Bond worked. Both services emerged from World War II, but neither were acknowledged to officially exist by the British government until 1992.
Evans also gave a blunt warning about the dangers from Iran, saying "a return to state-sponsored terrorism by Iran or its associates, such as Hezbollah, cannot be ruled out as pressure on the Iranian leadership increases."
CNN's Dan Rivers contributed to this report.