President Reagan announced last night that U.S. Marines will begin landing in Lebanon this morning. He said they will stay there "until all foreign forces are withdrawn" and the Lebanese government feels that it is again in control of its own country and "able to preserve order."

Asked how long the Marines might stay, the president answered, "I can't tell you what the time element will be." At another point, he said there was "no way to judge" when the Lebanese government will feel secure enough to let the American and other troops leave.

But Reagan said he expected the withdrawal of Israeli and Syrian forces in Lebanon to take place "rapidly" once the full 3,000-man peace-keeping contingent of U.S., French and Italian troops is in place.

The president noted that both Israel and Syria have declared themselves that they want to withdraw from Lebanon eventually. Syria has an estimated 30,000 troops in Lebanon, mostly in the Bekaa Valley in the eastern part of the country and with some troops in northern Lebanon. Israel has never said how many of its troops are there.

The president made no mention of remaining Palestine Liberation Organization fighters who presumably also must withdraw from Lebanon. The Israeli ambassador to the United States, Moshe Arens, suggested here yesterday that there were still thousands of PLO members in Lebanon, including some who had returned through the "back door" after having been evacuated from West Beirut earlier this month.

In his news conference last night in the East Room of the White House, Reagan described the U.S. troops' mission as meant to "give a kind of support and stability" to the efforts of the Lebanese government to restore order and "bring about a unified Lebanon."

"The Lebanese government will be the ones to tell us when they feel that they're in charge and we can go home," he said.

Asked if he feared that the United States could be dragged into a long, Vietnam-like entanglement there, Reagan said, "No, I don't see anything of that kind taking place at all." But he declined to answer what he called "hypothetical" questions about whether he would pull the Marines out if new fighting broke out.

Should it take a long time to help stabilize the new Lebanese government, the administration could run into provisions of the 1973 War Powers Act, which requires termination of the troop commitment after 60 days unless Congress authorizes continuation. Several lawmakers already have raised questions about this issue.

Late yesterday the United States finally persuaded Israel to remove its remaining military forces from the Beirut airport. The presence of those forces at the airport had been the stumbling block before Reagan would order the Marines ashore.

Pentagon officials said the first group of Marines are to land by sea at the port in West Beirut and are to be taken in trucks to the airport, south of the city, where they are to take control of the airfield. The rest of the Marines are to be flown directly to the airfield by helicopter from ships. An estimated 800 Marines will be in the first batch of troops to arrive today, with 400 more expected in a day or two.

In contrast to their initial duty in Lebanon last month, when the U.S. troops assigned to oversee the withdrawal of Palestinian forces from West Beirut were lightly armed and carried unloaded automatic weapons, the Marines going ashore today eventually will be supported with light amphibious armored vehicles and M60 tanks.

As Reagan did last night, administration officials also have been putting less emphasis on the possibility that the Marines will be withdrawn quickly in the event of shooting.

Officials have said that, while the Marines are not going to engage in combat or get involved in major hositilies, they will defend themselves if necessary and will not be withdrawn over isolated incidents of shooting. The idea is to avoid the impression that the force can be driven away easily.

Despite known White House frustrations in recent weeks with Israeli military moves in Lebanon and the actions of the government of Prime Minister Menachem Begin in Jerusalem, Reagan's tone toward Israel during the news conference was generally conciliatory and not critical.

This prompted one questioner to ask whether there had been any change in attitudes here toward the Begin government.

Reagan said his attitude toward Begin "isn't what some of you have said or written, that we are deliberately trying to undermine or overthrow the Begin government. . . . We have never had any thought of that kind."

The administration intends "to be doing business with the government of Israel and with Prime Minister Begin," the president said, "if that is the decision of the Israeli people." He emphasized that the United States remains "morally allied and obligated to the preservation of Israeli security."

The president said he didn't believe that Israel was trying to sabotage his long-range peace proposals for settlement of the Palestinian issue, but rather that both sides were staking out negotiating positions.

He also said a pending sale of additional F16 fighters to Israel is "still on tap," even though no formal notification has been sent to Congress. He acknowledged that it had not been sent to Capitol Hill because "frankly, in the climate of things going on, we didn't think it was the time to do it." But Reagan gave no indication that the sale would be held up indefinitely as a means to punish Israel.

Reagan said Israel "should understand, as we have come to understand in talking to Arab states," that, although Israel has been thought of traditionally as "a tiny country fighting for its life, its military power has become such that there are Arab states that now voice fear that it may be expansionist."

The president said he is not "less optimistic" about the chances for peace in the Middle East after the recent massacre of Palestinians in Lebanon, but, "I'm also not deluding myself that it's going to be easy."

Reagan said that what needs to be negotiated between Israel and the Palestinians ultimately is a trade of space for the Palestinians and security for Israel. And, he added, "This government will never stand by and see any settlement that does not guarantee the security of Israel."

Reagan has said that removal of all foreign forces from Lebanon was the essential first step in finding a solution to the Arab-Israeli problem. On Sept. 1 he proposed a plan which would create a self-governing authority for Palestinians in the Gaza Strip and West Bank in association with Jordan. Israel has rejected that plan flatly.