Security will be stepped up at railway stations, airports and ports as part of government attempts to tackle terrorism, Gordon Brown has announced.
There will be new security barriers, vehicle exclusion zones and blast resistant buildings, but air passengers will be allowed more luggage from 2008.
Rail travellers at large stations will also face having their bags screened.
The PM's statement came amid confusion over his security minister's views on detention limits for terror suspects.
In his wide-ranging Commons statement on national security, Mr Brown said that the failed bomb attacks in London and Glasgow Airport in June showed the need to ensure young people are not "radicalised" by extremists.
He outlined the creation of a new unit bringing together police and security intelligence to look not only at the "inner circle" of extremists but also at those at risk of falling under their influence.
The bulk of the statement covered security at public places such as transport hubs, which had been the subject of a review by ex-Admiral and current security minister Lord West.
Mr Brown said improved security would be installed at the country's 250 busiest railway stations, as well as airports, ports and more than 100 other sensitive locations.
"Additional screening" of baggage and passenger searches were planned at some large railway stations and other "sensitive locations", he said.
But the Department for Transport has stressed there are no plans to install permanent security scanners at railway stations - trials so far have involved portable or temporary systems, and sniffer dogs.
Mr Brown said guidance would be sent to thousands of cinemas, theatres, restaurants, hotels, sporting venues and commercial centres, as well as all hospitals, schools and places of worship to advise them on how to keep visitors safe against terrorism.
Ministers would work with architects and planners to encourage them to "design-in" better security measures in new buildings, such as blast resistant material, safe areas and traffic control measures.
Companies responsible for crowded places would be given updated advice on how they could improve resilience against attack, he said. About 160 counter-terrorism advisers will train civilian staff to watch out for suspect activity, ensure premises have adequate emergency facilities and make best use of their CCTV footage.
Improved facilities to screen baggage would allow airports to seek approval from 7 January to let passengers take more than one item of hand luggage on flights.
However, size restrictions on liquids and cabin luggage would remain.
The security budget, currently ?2.5bn this year, will rise to ?3.5bn in 2011, he said and the security service will double in size from 2001, when it had less than 2,000 staff - to more than 4,000.
He said tougher measures to deal with convicted terrorists would be included in the upcoming Counter Terrorism Bill and a new unit will be set up in the Charity Commission, to make sure charities are not exploited by extremists.
Talks on "repatriation arrangements" for terrorism suspects, already agreed with Jordan, Lebanon and Algeria, were underway with "a number of additional countries," he said.
The prime minister also outlined measures to counter the influence of radical fundamentalists in Britain's schools, universities, mosques, youth clubs and prisons, as well as on the internet. ( BBC )