(Reuters) - Regional lender Comerica Bank faces allegations it systematically mistreated millions of mostly disabled and elder customers and could pay penalties under new civil enforcement action, the U.S. watchdog agency for consumer finance announced Friday.

The Consumer Financial Protection Bureau said it was suing Comerica for failing 3.4 million "Direct Express" card holders -- who the agency said were primarily unbanked people receiving federal benefits -- by deliberately disconnecting their phone calls and charging them illegal fees.

In doing so, "Comerica boosted its bottom line at the expense of Americans living on a fixed income," Rohit Chopra, the agency's director, said in a statement.

The Dallas-based bank did not immediately respond to a request for comment.

The Direct Express program has operated since 2008 under a U.S. Treasury contract with Comerica and provides prepaid debit cards to recipients of federal benefits, according to the CFPB.

According to the agency, Comerica vendors intentionally dropped more than 24 million calls for customers before they could be connected with bank representatives.

Comerica also allegedly imposed illegal terms of service on consumers seeking to stop payments and illegally failed to investigate account problems.