DENVER -- Hours before a midnight deadline, a steady stream of Colorado senior citizens filed into offices Monday to sign up for the Medicare prescription drug program.

"It's been steady business, but not a madhouse," said Michael Fierberg, spokesman for the Denver office of the federal Centers for Medicare and Medicaid Services.

About 112,000 eligible Colorado seniors, or 20 percent of the total, had not signed up, but the exact number was uncertain because many may already have had coverage from employers, past employers or other sources, Fierberg said.

At the four enrollment centers in Denver, volunteers used a computer program to help seniors choose among 43 available plans for the combination of price and coverage that fit them best, Fierberg said.

After Monday's deadline, seniors who are not covered by a private prescription drug program will be shut out until January and face a 1 percent per month penalty on their premiums.

Fierberg said the penalty was added to encourage seniors to sign up now, even if they aren't currently taking medication. Insurance plans work best when the insured pool is large, he said.

The walk-in clincs will be open until 8 p.m. Monday and are at: Malley Senior Center, Englewood; St. Anthony Senior Life Center, Denver; St. Anthony North Hospital, Westminster; and New Hope Baptist Church, Denver.

In addition, Sen. Wayne Allard said his Denver office would remain open until 10 p.m. Monday to assist seniors.

"Please contact my Denver district office … so my knowledgeable staff, who are very familiar with the program, can help you sign up and answer any questions,' Allard said in a prepared statement.

Allard's Denver-area office is located at 7340 E. Caley Ave., Suite 215, Englewood, Colo. The phone number to call is (303) 220-7141.