"As New York City's Chief Financial Officer and the Chief Investment Advisor to the City's Pension Funds, I am particularly troubled by reports that Wal-Mart engaged in chilling and truly outrageous surveillance activities," Thompson wrote. Copies of the letters were posted on the comptroller's Web site.

Wal-Mart could not immediately comment on the story. The SEC declined to comment.

Last week, a report in The Wall Street Journal said an internal Wal-Mart security group was asked to investigate the potential threat of those submitting proposals to its June shareholder meeting.

Following the article's publication, Tom Hyde, Wal-Mart's top legal officer, sent a letter to shareholder proponents saying that while the article implied that the company had initiated an intrusive "threat assessment" of shareholders who submitted proposals for inclusion in Wal-Mart's annual proxy statement, that was not true.

"In spite of a January 2007 memo referenced in the article, there were no inquiries made with respect to the proponents of shareholder proposals," he wrote in the letter, a copy of which Wal-Mart provided. "Given the nature of the matters proposed and our familiarity with the individual proponents, the request contained in the memo was not acted upon."

The New York City Pension Funds represent nearly 600,000 active members, beneficiaries and retirees and are valued at nearly $100 billion, according the Web site.

Wal-Mart shares fell 24 cents to $47.70 in morning New York Stock Exchange trade.