Americans face a Monday deadline for enrolling in health insurance after last-minute efforts over the weekend produced long lines, extra security and hours of waiting across the country.

HealthCare.gov, the federal web portal for purchasing coverage, was up and running after sunrise on the East Coast, but early visitors saw messages that the site was down for maintenance. Obama administration spokesman Aaron Albright said the site's "regular nightly maintenance" was extended for what the Department of Heatlh and...