The program is modeled after one begun three years ago in New York that has withstood legal challenges. However, experts said it is difficult to measure the effectiveness of such searches, beyond assuring the public that police are being vigilant. New York officials declined to say what they have found in their searches; none of the other transit systems conducting random searches have found any explosives, officials said.

Metro officials said the program was not in response to a specific threat but prompted by increased security concerns before next week's election and the inauguration as well as by the Sept. 11, 2001, terrorist attacks and later bombings of commuter trains in Madrid, London and elsewhere.

Although Metro police said the program will begin immediately, they would not say which of their 86 rail stations or more than 12,000 bus stops would be subject to inspection on any given day. On some days, there might be no inspections, or there might be several. Fifteen officers have been trained to perform searches, and more will be trained, officials said.

Checkpoints will be set up at Metro facilities, and passengers will go through inspections before entering a rail station or boarding a bus. The random searches will focus on detecting explosives, and it is likely that some riders will have their bags inspected before next Tuesday's election, officials said.

Metro, the second-busiest subway system in the country, after New York's, carries more than 1.2 million passenger trips on a typical weekday.

"We realize that all Americans everywhere are at some risk from terrorism, and that those of us who live and work in the region of the nation's capital face increased risks," Metro Transit Police Chief Michael Taborn said at a news conference yesterday.

U.S. intelligence agencies have long warned that the weeks just before an election and immediately after are considered a "zone of vulnerability" for the country. The teams tasked with helping the winner of next week's presidential election transition into office also have been warned about the heightened chances of attack. Officials note that the March 11, 2004, Madrid train bombings took place three days before general elections in Spain.

The Metro searches will take place only when Transit Police determine that circumstances -- such as an elevated threat level -- warrant heightened vigilance. No advance notice will be given, but just before inspections begin, Metro police will post signs alerting riders. Inspections will be conducted by five to eight Transit Police officers and a police dog trained to sniff for explosives. Officials said searches would last eight to 15 seconds.

Transit Police will only inspect areas of bags that are capable of concealing explosives. Police will not be viewing the content of papers or other reading material. But if illegal items such as drugs are found, they will be confiscated as evidence, and police will cite or arrest the individual. Those who refuse to have their bags searched will not be allowed to enter. Transit Police will not arrest people who refuse to have their bags inspected.

In the searches, Transit Police will randomly choose a number, such as 17. Then they will ask every 17th rider with bags to step aside for an inspection before boarding a bus or entering a rail station. If others are acting suspiciously, Transit Police have the right to stop a person not selected for inspection.

In an online discussion yesterday on washingtonpost.com, commuters expressed concern and confusion. Few thought the program would make them safer. There was also confusion about how police would handle the searches.