TUNIS, NOV. 8 -- Zine Abidine Ben Ali, who took power yesterday from the ailing Habib Bourguiba, has promised Tunisians a new era. But as the country emerges from the shadow of its perennial Supreme Combatant, it is unclear what the new era will bring.

Bourguiba, who had governed since independence from France in 1956, was the architect of modern Tunisia. He infused its people and institutions with his own definitions of politics, Arabism and Islam. Now that he has been declared incapacitated and replaced at age 84, Tunisia has lost the man who in large measure gave the country its national personality.

Bourguiba's departure from the presidency, although bloodless and broadly applauded here, thus has opened the way for new directions and new personalities that could change Tunisia's easy-going ways and alter its westward-looking face.

Ben Ali, a retired Army general with training in France and the United States, immediately indicated he will seek to continue Bourguiba's tradition of friendly relations with the West and moderation in North African and Arab affairs. Although he has made a career of security and was interior minister during harsh repression under Bourguiba, Ben Ali was seen as an advocate of political opening and moderation in Tunisia's internal troubles as well.

In that spirit, he accompanied his takeover with pledges that opposition parties will be allowed to participate more fully in political life, something Bourguiba would not tolerate. Although he did not specifically call for new elections, suggesting he intends to stay in office at least until the next scheduled vote in 1991, Ben Ali also declared that Bourguiba's presidency for life was unacceptable and that people must participate in choosing their ruler.

The ruling Destourian (Constitutional) Socialist Party, however, traditionally has been a political machine for enforcing Bourguiba's personal rule. Ben Ali has built his career mostly within the military and is a late-comer to the top ranks of the party. As a result, Ben Ali could have a difficult time asserting himself as the organization's new czar. This opens the way for party activists to seek new leadership and policies as ways of gaining popular support or competing for power with Ben Ali.

One measure of how extensively Bourguiba dominated political life here was the calm assertion by a young man in the center of Tunis last night that the aged president actually engineered his own ouster, outfoxing his opponents yet again.

By this afternoon, Bourguiba's pictures were coming down and being replaced on street corners and in government offices by freshly printed official portraits of Ben Ali. But he dusty outlines of where Bourguiba's photos had hung for years remained in some places, reminders that Ben Ali will have a hard time taking his place.

"Ben Ali may try to speak in terms closer to the population at large, which is the Arab side of the culture," a diplomatic source suggested.

The strongest opposition currents, if allowed to grow under the new leadership, also are likely to advocate policies more closely oriented toward the Arab world or Tunisia's North African neighbors and less easily influenced by the West, well-informed observers predicted.

A fundamentalist group that the government has forced underground in recent years, the Islamic Tendency Movement, has generated substantial support with calls for strict adherence to Moslem ways along the lines of the Islamic revolution in Iran. The religious militants have drawn much of their appeal from general opposition to Bourguiba, however, and some Tunisians have suggested that in open political battle, the cry of Islamic fundamentalism would weaken.

The Islamic Tendency Movement endorsed the takeover today and called for new legislative elections and freedom of action for "mass organizations," presumably including their own. There was no immediate response from the new government.

Bourguiba's determination to eradicate the Islamic militants played a major role in Ben Ali's decision to push him aside, according to informed Tunisian sources. Khemais Chamari, head of the Tunisian Human Rights League, said Bourguiba last week ordered a new closed-door retrial beginning Monday for a group of extremists who already were tried and sentenced in September.

Chamari said the new prime minister, Hedi Baccouche, told opposition political leaders that Bourguiba sought to have at least a dozen of the same extremists hanged by the end of this week, leading Ben Ali and several other officials to conclude they had to move against him to prevent popular unrest.

Bourguiba was violently opposed to the Islamic fundamentalists because they represented a threat to his power. They also challenged the moderate version of Islam that he ushered in for his country soon after independence. This tradition, including women's emancipation from previous interpretations of Moslem strictures, was closely tied to Bourguiba's personal rule and one of his proudest achievements.

One of Tunisia's historic moments, for example, came in 1964, when Bourguiba saw the Islamic month of fasting, Ramadan, as an obstacle to economic development. He shocked traditional Moslems by drinking a glass of orange juice on national television and urging citizens to break the fast. Following that dramatic gesture, Bourguiba set up a society where Tunisians can sip an evening beer even as the muezzin's call to prayer floats over the city.

The main traditional political opposition, the Movement of Democratic Socialists, also has taken positions more closely tied to the Arab cause than Bourguiba's own policies. The movement's leader was arrested for leading an unauthorized demonstration against the U.S. bombing of Libya in 1986.