A longtime, prominent Seattle developer is facing a mounting string of legal actions as he struggles to pay off millions of dollars in loans at dozens of banks across the Pacific Northwest.

Michael Mastro Sr., for years among the most successful commercial developers in the Puget Sound region, is quickly becoming a source of concern at banks — both because of their direct exposure and because of what his troubles say about the potential pain still ahead in the commercial real estate market, according to people familiar with the matter.

His plight points to a larger trend. Banks that have been battered by residential losses are now facing more difficulties in the commercial real estate game as well.

Mastro, and his company, Mastro Properties, owe about $500 million to more than 25 banks in Washington and Oregon, including local banks, such as HomeStreet Bank, and national lenders such as Wells Fargo and Bank of America, according to a Mastro company associate and other people familiar with the matter.

Mastro values his assets at more than $600 million — more than enough to cover his debts — and he expects to recover. But court documents and people familiar with the matter indicate he presently appears to lack the cash flow to make loan payments.

On one $30 million construction loan from Bank of America, for example, Mastro is accruing unpaid interest of about $5,500 a day, according to a recent complaint filed by the bank against Mastro.

While Mastro is not the only Puget Sound region developer suffering as a result of the housing market’s downturn, his broad reach across dozens of financial institutions extends farther than many other developers.

“He’s one of many people who are in trouble but he’s probably had bigger trouble than anyone else. It’s just one indicator of the trouble that’s going on in real estate,” said Michal Makar, executive vice president for debt and equity finance at the Seattle office of CB Richard Ellis, a real estate services company.

Mastro’s troubles offer a stark example of the real estate downturn’s brutal effect on developers and the large number of banks that gambled on the market’s rise. His situation also represents what is shaping up as the next wave of problems for banks: commercial real estate.

Already reeling from the collapse of the housing market, lenders now face potentially huge exposure to the deteriorating market for commercial property. In turn, developers are finding financing more difficult to obtain, making it harder to fund development projects and buy and sell property.

Mastro’s enviable portfolio, sterling reputation and relationships with many financial institutions through a long, successful career, helped him amass millions of dollars in loans and assets, according to people who have worked with him.

“He always paid you back,” said a person familiar with the situation. “That’s also why everyone got in bed with him so much — he’s always been true to his word.”

But in recent months, Mastro has raised alarms by indicating he is unable to keep up with all of his interest payments, according to court documents and people familiar with the issue.

Some banks already have taken action against Mastro, filing a number of lawsuits and winning court judgments of $1 million or more after he allegedly failed to make payments on loans. Others have recently moved their Mastro loans and credit lines to a secured position, by insisting the developer back up his loans with property.