American Telephone & Telegraph Co. plans to raise$400 million in seven-year debt on the Eurobond market in its first venture in the European financial markets, the company announced yesterday.

At the same time, AT&T Chairman Charles L. Brown said that because of a downturn in the economy, AT&T will reduce its 1982 construction program from a projected $18.9 billion to at least $18.3 billion. Such a move will reduce AT&T's need for external financing by $500 million so that AT&T's short- and long-term debt and equity needs would be in the range of $4 billion to $4.5 billion for 1982.

AT&T's venture into the Eurobond market comes as AT&T is trying to attract overseas investment. Just four months ago, the company hosted a number of European financal leaders visiting the United States.

The proposed sale by AT&T's 10-month-old international subsidiary, AT&T Overseas Finance N.V. , will be managed by an international underwriting group led by Credit Suisse First Boston Limited. AT&T hopes to list these securities on the Stock Exchange in London, which has been listing AT&T's common shares since January.

By law, the bonds cannot be sold in the United States.