After the Tunisian revolution and the emergence of a Hezbollah-backed government in Lebanon, Israelis are confronting another jolt to the system as mass protests rock Egypt, the partner in Israel’s oldest and most important Middle East relationship.

While the recent upheavals have not been about Israel, they could have a potentially momentous impact on its future. Yet Israel, often a major player, now finds itself in the less familiar, and somewhat unnerving, role of spectator.

“When we say we are following events closely,” said an Israeli official, who insisted on anonymity because of the delicacy of the diplomatic situation, “that is the truth. There is not much else we can do.”

Israel has a special stake in Egypt’s stability. The two countries share a long border and signed a historic peace treaty in 1979, a cornerstone of the regional balance that has endured more than 30 years.

Though the peace, Israel’s first with an Arab partner, has remained cold — Egyptian civil society still boycotts Israel — the relationship is viewed here as critical. Israel’s prime minister, Benjamin Netanyahu, confers regularly with Egypt’s president, Hosni Mubarak; they met most recently on Jan. 6 in the Sinai resort of Sharm el Sheik for what another official described as strategic discussions.

“Egypt is not only our closest friend in the region,” Binyamin Ben-Eliezer, a veteran Israeli politician and former defense minister known for his close ties to senior Egyptian officials, told Army Radio on Wednesday, “the cooperation between us goes beyond the strategic.”

Israeli officials and analysts said they believed that Mr. Mubarak’s government was strong enough to withstand the protests, at least as long as it had the backing of the Egyptian Army.

But with Mr. Mubarak, who came to power in 1981, now an ailing octogenarian, Israelis were in any case looking ahead to a transition of some sort in Egypt, amid a sense of a shifting regional equilibrium.

Israelis speak of two arcs in the region — a northern, Iranian-oriented one including Iran, Syria and now Lebanon; and a more moderate, southern arc spanning North Africa, Egypt, Israel, the Palestinian Authority, Jordan and the Persian Gulf states.

“We see the northern arc growing in strength,” said Oded Eran, director of the Institute for National Security Studies at Tel Aviv University and a former Israeli ambassador to Jordan, “and the southern arc in a very volatile period.”

“While we should all congratulate the forces calling for more democracy, if this is the case,” he added, noting that the opposition in Egypt includes Islamic fundamentalists, “for now, the effect is destabilizing.”

Israelis were not yet envisaging a future without the peace treaty with Egypt. Mr. Eran said that almost any government in Egypt would want to maintain the pact, even at a low profile, because so much is hinged on it, including Egypt’s relations with, and aid from, the United States.

At least in the short term, Israelis did not see a need for panic. At the same time, officials here were cautious about making long-term predictions. After Mr. Mubarak leaves the stage, one said, “We have no idea what will happen.”

It was unusual to see the front pages of the Hebrew newspapers, which mostly obsess about domestic issues, taken over by foreign news. But then, the news was not truly foreign, with Lebanon and Egypt neighboring Israel to the north and south.

Closer to home, the leaked documents published this week by Al Jazeera showing Palestinian concessions to Israel on sensitive issues during past negotiations has posed a new challenge to the Palestinian Authority leadership based in the West Bank.

Yet the West Bank remained mostly calm. One Palestinian analyst, Mahdi Abdul Hadi, director of Passia, an East Jerusalem research institute, attributed the response to what he called the crushing effect of decades of Israeli occupation. Others say the Palestinians are enjoying the relative stability and growing prosperity in the West Bank after years of violence.

The regional turmoil comes at a time of stalemate in the Israeli-Palestinian peace process, which has raised concerns about a possible deterioration of the local situation. Some analysts said that the leaks would make it harder to bring the sides back to peace talks and that the Palestinians would harden their positions.

Still, there were Israelis who suggested that the tumult in the region and the uncertainty of the future of its leaders would make it harder for Israel to take risks for a peace agreement or to make far-reaching decisions