One British security source said two of the suspects were Indian, the rest were Middle Eastern and "quite a few" were doctors -- a contrast with recent British conspiracies led by "homegrown" militants, often with modest academic backgrounds.

Two of those arrested worked at hospitals in England, one was a doctor in Scotland and Australian police also detained an Indian doctor, Mohamed Haneef, under counter-terrorism laws. Police sources said the other suspects also had medical links.

The discovery of two car bombs primed to explode in London's bustling theatre and nightclub district last Friday put a city already attacked by four suicide bombers in 2005 on edge.

When a fuel-laden jeep rammed into a Scottish airport the next day, Britain's threat level was raised to its highest level, "Critical", meaning more attacks might be imminent.

Two more men were arrested on Tuesday in northern England but police said it was too early to link them to the plot. A local newspaper said they were held after 10 gas canisters, similar to those found in the cars, were delivered to an estate.

The attacks pose a stern test for Prime Minister Gordon Brown, a Scot who replaced Tony Blair last week and has come under some pressure to withdraw British troops from Iraq.

Britain has seen a marked increase in terrorism-related attacks since the Sept. 11 strikes on the United States and its decision to join U.S. forces in invading Iraq in 2003.

Muslim leaders praised the government on Tuesday for its "calm and reassuring tone" in handling the crisis and said they recognised there was a problem of extremism in their community.

"Those who engage in such murderous actions and those that provide support for them are the enemies of all, Muslim and non-Muslims, and they stand against our shared values in the UK," Muhammad Abdul Bari, secretary general of the Muslim Council of Britain, told a news conference in London.

In its statements, the government has not singled out Britain's large Muslim community, nor called for tough new security measures -- a contrast with Blair's administration.

However, fears of a backlash against Muslims in Scotland rose on Tuesday after attackers rammed a car into an Asian-owned shop in Glasgow and set it ablaze. The shop was burned out.

Fearing more attacks by Islamist militants, police have banned cars and other vehicles from approaching airports.

With Britons on alert and security measures beefed up at transport hubs, there have been a number of security scares.

A terminal at London's Heathrow Airport was evacuated as police investigated a suspicious package, and an object left in a busy west London street was blown up by police.

In Scotland, police have carried out four controlled explosions at a hospital linked to at least one of those arrested and at a mosque in the biggest city, Glasgow.

Police said they were still looking for other suspects.