Capitol Police tightened security procedures across the Congressional campus Thursday morning — stepping up sweeps for explosives and vehicle searches — in response to a series of bombings on London’s subway and bus system.

A police spokesman said the increased measures include supplementary patrol units and an increased law enforcement presence on Capitol Hill beginning Thursday morning, shortly after the incidents occurred in London.

“We’re going to increase presence” on the grounds, said Officer Michael Lauer, a department spokesman.

According to a knowledgeable source, those efforts also include round-the-clock presence of the department’s hazardous devices unit, as well as periodic sweeps of Congressional buildings.

“Public areas inside the buildings are being checked hourly for anything suspicious,” the source said. Capitol Police are also expected to sweep nearby Metro stations for explosives throughout the day, the source said.

Additionally, Lauer said law enforcement officials will give extra attention to anyone carrying a large item, such as a backpack, anywhere on the Capitol grounds.

“Any person carrying any type of backpack or suitcases will be screened by our uniformed officers,” Lauer said. Officers will make a “visual inspection” of anyone with those designated items, Lauer said, and could elect to conduct a further physical inspection if there is reasonable cause to do so.

Existing regulations already prohibit visitors from bringing large bags — as well as stun guns, martial arts weapons, firearms, fireworks, knitting needles, aerosol and non-aerosol sprays, cans and bottles, food and beverages, knives, razors, box cutters, mace, and pepper spray — into the Capitol, although Members and staff are not subject to the same restrictions.

Capitol Police have also begun to screen all “large vehicles” such as tour buses, limos and other automobiles entering the Congressional campus during rush hour periods in the morning until 10 a.m., and Thursday afternoon between 4:30 and 6:30.

(Some vehicles including tractor-trailers and other large trucks are already prohibited from entering the Capitol grounds under existing regulations.)

“Any large vehicles coming out would be stopped to be checked by canines,” Lauer said. However, he added, the department does not intend to reinstitute the full-scale vehicle checkpoints that it set up for several weeks in August 2004 in response to government warnings about possible terrorist attacks on financial areas in Washington, D.C., New Jersey and New York.

Public tours of the Capitol will not be affected by the increased security, however, and will continue on the regular schedule, according to the Capitol Guide Service.

Additionally, Homeland Security Secretary Michael Chertoff announced late Thursday morning that the federal terror alert level would be raised to orange, the second highest level, but said the warning would apply only to mass transit, such as regional and intercity passenger rail, subway and metropolitan bus systems.

While Chertoff acknowledged there is “no specific credible information suggesting an imminent attack” in the United States, he said the Homeland Security Department will work with the Transportation Department and other federal, state and local agencies “to take all necessary precautions to increase” security of those transportation systems.

Those measures could include an increased law enforcement presence, the use of bomb-detection dogs and added barriers around transportation hubs.

“We ask the public to remain alert and to report any suspicious activities,” Chertoff said, adding that federal officials are not advising the public to avoid public transportation.

According to information provided by the House Emergency Communications Center to staff, Capitol Police state there is likewise no evidence of a specific threat against the National Capital Region or the Congressional campus.