A new angle for a venerable Paris landmark; A glass pyramid for the Louvre

President Francois Mitterrand wants to leave his architectural mark on Paris. Among those helping him to achieve his aim is the American architect I. M. Pei. The President's approval of Mr. Pei's design for a glass pyramid in the entrance courtyard to the Louvre museum has stirred controversy in French artistic circles.

But Mr. Mitterrand has not allowed himself to be swayed by criticism of the pyramid for clashing with the Louvre's 16th- and 17th-century architecture. Preliminary work has already started on the pyramid project, which is due to be completed in 1987.

Visitors to the Louvre have to wend their way around brightly decorated wooden fences erected to close off the site.

The pyramid is part of a bigger project to make the whole of the huge Louvre building, lying at the end of the Tuileries Gardens beside the Seine, into a museum. At present large sections of the building are occupied by the French Finance and Budget ministries.

Under Mitterrand's plan, the ministries will be moved to a new office complex in a redeveloped warehouse site along the Seine. The additional museum space will allow the Louvre to show thousands of treaures currently kept in storage for lack of exhibition room.

As part of the redevelopment of the museum, a new entrance will replace the current cramped way into the museum. The plan is to use a broad underground passageway starting in the courtyard and fanning out to provide access to the different sections of the museum.

Support services for the museum will be located underground. So will a collection of restaurants, shops, and video viewing rooms for the public.

Pei's pyramid will cover the entrance to this underground complex. Mitterrand was eager to have a striking new structure to mark the new way into France's biggest museum. Pei himself describes his design as being ''a luminous structure-symbol.''

The glass pyramid will stand over 60 feet high. By day, its transparent sides will allow a clear view from the Louvre up the Champs-Elysees to the Arc de Triomphe. At night, it will be illuminated. When the sun shines, it should provide a sparkling new focal point for one of the most visited parts of Paris.

This has not stopped critics from rounding on the Pei design. Some fear that it will obstruct the famous view from the Louvre courtyard to the Arc de Triomphe. The project's backers insist this will not be so.

Other critics predict it will disturb the classic architectural lines of the Louvre. But Pei's model of the pyramid keeps a clear distance between the base of the new structure and the pathways leading to the Louvre around the edge of the courtyard on the Tuileries side of the building, known as the Cour Napoleon.

Opposition to the project is based on the feeling that a modern building standing so prominently in such a celebrated site will clash with the dignified, Louis XIV environment of the Louvre and the Tuileries Gardens.

The French commission for historic monuments voted against the project, 60 percent to 40 percent. Former Culture Minister Michel Guy opposed the design.

But Mitterrand liked it and so does France's current culture minister, Jack Lang. Mr. Lang compares the project's critics to those who opposed the modernistic Pompidou Culture Center in Paris, which is now one of France's top attractions for visitors.

Another supporter is the mayor of Paris, Jacques Chirac.

The redevelopment of the Louvre and the Cour Napoleon is due to cost 2 billion francs ($215 million). Despite his orders to ministers to hold down government spending in recent momths, Mitterrand intends to push through the Louvre project as part of a broader plan to redevelop Paris.

Work is proceeding on a museum devoted to the 19th century on the left bank of the Seine nearly opposite the Louvre.

Big new sites are being developed on the edge of the city at Bagnolet and La Villette to house music centers and a science and industry museum. A ''popular'' opera house is due to be built on the Place de La Bastille.