Morgan Stanley, Treasury's agent, will have discretionary authority to divest the shares through a trading plan that involves 12 other securities brokers. That plan will run until the end of the year.
Treasury injected $45 billion in Citigroup in late 2008. The financial services giant repaid $20 billion of that money last year, and the remainder was converted to common stock, which gave taxpayers a roughly 27 percent stake in the company.
To date, Treasury has liquidated 4.1 billion of its 7.7 billion Citigroup shares, generating $16.1 million in gross proceeds. The new round of sales will cut its stake in the company to approximately 7 percent.
Citigroup's shares closed Monday at $4.17. At that price, the 1.5 billion shares being marketed would bring nearly $6.3 billion.
So far, Treasury has recouped $41.6 billion of its Citigroup investment, through repayments by the company, dividends , the sale of its common stock and the sale of other securities.