LONDON -- The military-style attack in Paris has made clear that Europe faces an evolving, ever-more complex terror threat no longer dominated by a few big players.

It's not just al Qaeda, or Islamic State. It's not just the disciples of some fiery, hate-filled preachers.

Instead, security experts say, it's now an Internet-driven, generalized rage against Western society felt by radicalized Muslims that can burst into the open at any time -- with a slaughter in Paris, an attack on a Jewish Museum in Belgium, or the slaying of a soldier in the streets of London.

This evolving hydra-headed beast bedevils security chiefs, who have to deal not only with al Qaeda planners looking for another 9-11-style hit but also with, as in Paris, well-trained, well-armed killers intent on avenging perceived insults to their religion by gunning down journalists.

In a rare public speech, Andrew Parker, director of the domestic British security service MI5, said Thursday that thwarting terrorist attacks has become more difficult as the threat becomes more diffuse.

It is harder, he said, for agents to disrupt plans of small groups or "lone wolves" who act spontaneously, with minimal planning but deadly effect.

"We believe that since October 2013 there have been more than 20 terrorist plots either directed or provoked by extremist groups," he said, citing deadly attacks in Europe, Canada and Australia. He said security services have stopped three potentially lethal terrorist plots inside Britain alone in recent months.

"The number of crude but potentially deadly plots has gone up," he said, warning that small-scale plots carried out by volatile individuals are "inherently harder for intelligence agencies to detect."

The individuals are not part of disciplined, sophisticated networks, he said, and often act with little or no warning.

Already some 600 Britons have gone to Syria to join extremists there, with most embracing Islamic State, Parker said. Some 550 Germans have done the same, with about 180 known to have returned, including a hard core of about 30 who are judged to be extremely dangerous, according to German Interior Minister Thomas de Maiziere. About 1,200 French citizens have left for Syria, including about 400 still in the war zone and 200 on their way, French Interior Minister Bernard Cazeneuve said last month.

Parker said they have learned how to hate and how to kill.

Concentrating solely on these volatile individuals wouldn't work, he said, because at the same time rival al Qaeda and Islamic State groups inside Syria are trying to orchestrate broader attacks in Britain and Western Europe.

Open societies everywhere have difficulty protecting against terrorism, whose perpetrators are aided by the very freedoms and openness that they often despise. But in Europe, several factors further complicate the situation.

The main one is a large Muslim population in many countries -- France first among them, but also Belgium, Sweden, Germany, Britain, and even Spain and Italy. The size of these communities enables the radicals among them to better hide.

The issue is compounded by the fact -- only recently the source of angst in Europe -- that many immigrants are not well-assimilated into Western society. While most immigrants are law-abiding and non-hostile, it seems that many have not absorbed its liberal values, including freedom of expression up to and including satire of religious figures. This creates an atmosphere in which radicalism can survive and sometimes thrive.

Magnus Ranstorp, a terrorism specialist with the Swedish National Defence College, said a new generation of Muslim youths has grown up in Europe's cities in the post 9-11 era and has to a degree embraced the al Qaeda view that the West is at war with Islam -- first in Afghanistan, then Iraq and now in Syria as well.

At the same time, he said, the Islamic State's brazen proclamation of a caliphate has caught the imagination of many young European Muslims, who want to go to Syria to join the battle and then bring it back home.

"The sectarian tensions in the Middle East are mirrored in our cities in Europe," he said. "There is more strident activism in Muslim communities."

He said many Muslims feel segregated in disadvantaged communities on the fringes of major cities and are willing to fight back.

"There is a much sharper polarization of society," he said, citing the corresponding rise of right-wing, anti-immigration political parties opposed to the growth of Islam in Europe. "The people carrying out the violence work in small groups but they all join up and know what direction they are travelling in. They are very clear on the goal. The caliphate provides that common purpose, that unity, that momentum."

The law-enforcement challenge is exacerbated by the free movement of people that is a cherished ideal of the European integration project. It is an item of faith that open borders will spur trade, job creation and spread prosperity.

But it also makes it much easier for anyone with criminal intent and an EU passport to cross borders to carry out an attack -- as happened in May when a Frenchman linked to the Islamic State group in Syria crossed into Belgium and killed four people at the Jewish Museum in Brussels.

U.S. Rep. Adam Schiff, a member of the House intelligence committee, said U.S. officials are making a strong effort to track Americans who have gone to fight in Syria and Iraq. But the challenge for European officials is much more daunting, he said.

"It's tough though, particularly when we don't have great intelligence in places like Syria to identify what's happened to Americans who have gone overseas to fight," he said. "Very opaque and difficult to track. That problem is magnified a hundred times in Europe, where people can travel freely with a passport."

Britain took unilateral steps Thursday to tighten up its border checks at seaports and train stations, and Spain raised its terror threat level, not because of a specific plot, but because of a general sense that all of Europe -- not just France -- was at heightened risk since the attack in Paris on the newsroom of the satirical weekly, Charlie Hebdo, that left a dozen people dead.

Spain also stepped up security Thursday at transportation hubs like airports and train stations, nuclear power plants, energy networks and water sources.

"The current international scenario means we can talk about a generic threat that is shared by all Western countries in general," said Interior Minister Jorge Fernandez Diaz.

He said the rivalry between the two main terror organizations-- which are vying for primacy in Syria and elsewhere -- is being felt in Europe.

"There is a clear battle between al Qaeda and the Islamic State to become terror leaders. And this increases the risk of attacks," he said.

Pointedly refusing to use Islamic State's chosen name in his address Thursday, Parker said the group's effective social media strategy has allowed it to spread its "message of hate directly into homes across the United Kingdom."

He said the group poses a three-pronged threat: It has murdered innocent Britons inside Syria, it is using Syria as a base for directing terrorist attacks against Britain, and it is using its sophisticated propaganda to provoke Britons to carry out attacks at home.

The brothers suspected in the Charlie Hebdo killings were known to France's intelligence service and were on the U.S. no-fly list, yet authorities were unable to prevent the attack, in part because the planning group involved may have been quite small and operating under the intelligence radar. The same was true of the two al Qaeda-inspired British extremists who hacked to death soldier Lee Rigby on a busy London street in May 2013.

Peter Neumann, director of the International Center for the Study of Radicalization at King's College London, said the smaller attacks seen of late reflect a change of strategy among jihadi groups, who have previously harboured ambitions to create incidents as big as the Sept. 11 attacks on the United States or the subway bombing attacks on Britain in July 7, 2005.

"Now what has happened since last year is that everyone has realized that you can cause as much terror if you do very small attacks that do not require you to build a bomb," Neumann said. "They've been incredibly effective."

He said there will be other similar attacks in the future.