Her Serene Highness Princess Caroline of Monaco became Madame Philippe Junot yesterday in the eyes of the law of her tiny Mediterranean principality. The private, 5:30 p.m. ceremony in the 16th century Throne Room of Monaco's palace ended the first phase of one of royal society's most controversial weddings in years.

This morning in the palace chapel 500 representatives of European aristocracy, high society and cafe society will watch as the pair solemnize their vows in a Roman Catholic ceremony. Only then will the marriage be official in the eyes of their church, only then may their honeymoon begin.

For yesterday's civil ceremony, Caroline, whose taste normally runs to blue jeans or flashy disco dress, appeared demure and girlish in a pillbox hat and ice-blue georgette Dior dress. "Oui," she answered softly when Louis Roman, president of the Monaco State Council, asked whether she took Junot as her husband. Wearing a dark business, suit, the bridegroom simply said "Oui."

Before marrying the pair, Roman asked two questions: Did the couple have a marriage contract and did Caroline wish to keep her Monegasque citizenship? "I will retain Monegasque citizenship," the princess replied.

Only 50 guests, largely the immediate families of the bride and groom, attended the ceremony, which, like today's religious ceremony, was covered only by pooled press arrangements. Monacan television showed the event.The guests had taken their seats as Princess Grace, in a pale crepe gown by Dior and matching face veil, and Prince Rainier entered followed by their other children.

Then Caroline and Junot filed in to take their seats opposite Roman.

Later, Roman told reporters that the hands of both the bride and the groom had trembled as they signed Monaco's official marriage register. Signing right after them were Prince Rainier and Princess Grace, Junot's parents and Caroline's younger sister Princess Stephanie. Then her brother, and the heir to the throne, Prince Albert signed as one of the two official witnesses, followed by Caroline's COUSIN, Grace LeVine of Philadelphia.

The entire ceremony, including Roman's reading of the couple's marital obligations, lasted just 20 minutes.

Immediately after the ceremony, the newlyweds and their families stepped onto the balcony of the palace's interior courtyard where the invited citizens of Monaco had gathered. They greeted the couple with applause, and the clapping was acknowledged with timid waves by the two.

The 4,500 native-born Monacan citizens, who are called Monegasques and are only a fraction of the principality's 23,000 permanent residents, received individually inscribed, engraved invitations. They came in their Sunday finest.

Junot and Caroline descended the broad stone stairs of the palace to mingle with the crowds. At the foot of the stairs, the mayor of Monaco presented Caroline with a pair of Cartier earrings bought by subscription with money left over from contributions by the population for a 200-piece set of sterling silver.

The assembled Monegasques included a cross section of the principality's occupations - doctors, teachers, office workers, pastry makers and croupiers. Citizens questioned by reporters expressed only good feelings. "We have a beautiful little princess," said an elderly woman in a floral-print dress. One woman looked startled when asked her opinion. "But, I am a Monegasque, sir," she replied. The tone and expression seemed to imply that if she had any criticism she would keep it to herself.

A middle-aged man expressed special admiration for his prince's speech. What did he say "I don't remember, but it was very good. There are people who speak to say nothing, but that's not the case with him."

Shortly after the reception for the Monegasques began the Junot family slipped away quietly. Philippe's mother, wearing a broad-brimmed pale green hat to match her green blouson dress, dabbed at her eyes as she entered the waiting black limousine, crushing the large hat against the doorway as she entered.