Security has been increased and the country is on its highest state of alert - critical - warning that a terror attack could be imminent.
Prime Minister Gordon Brown told British television Sunday that the country faces what he called a "long-term" threat from al-Qaida-linked extremists.
Two unexploded car bombs were found in central London Friday - one near a nightclub and another that was left in an underground parking garage. That vehicle was towed to an impound lot, where police found gas canisters, gasoline and nails inside.
On Saturday, two men rammed their vehicle into the terminal at Glasgow airport, causing the vehicle to burst into flames. The men were detained. One of them remains hospitalized with severe burns.
Police in northern England have arrested three people.
Some information for this report was provided by AP and Reuters.