A little-notice administrative order issued by the Lebanese government last week is apparently going to disrupt the travel patterns, business practices and living styles of much of the Arab world.

As of April 10, all foreigners except Syrians will need entry visas before they can visit Lebanon.Although Lebanese consular officials abroad will have the authority to issue some visas on short notice, most applicants face long delays and waits of several weeks before they can travel to Lebanon, government officials say.

Up to now, amost any foreigner could obtain an entry permit at the Beirut airport or at the Syrian border, and Arabs could enter the country at will - as they did, by the untold thousands every year, for vacation, study, medicl care, politics, arms trafficking, terrorism, the hashish trade, currency speculation and legitimate business.

Since the end of the 1975-75 civil war, Lebanon has partly regained its status as a hub of banking and politics, and flights into Beirut are jammed. Now, however, airlines flying to Lebanon are being instructed not to accept passengers who do not have valid entry visas.

The order requiring visas was officially described as temporary, but there is no telling how long it will remain in effect.

Lebanese consular officials say that most visa applicants will be required to submit photostats of their passports, the name and address of a sponsor in Lebanon and proof of a valid reason for their visit.

This material will then be sent to security officials in Beirut before the visa is approved - a process that consular officials say is likely to take many weeks, given the administrative chaos here and the fact that hundreds of thousands of Lebanese are not actually living at their official addresses.

"What if somebody wants to come visit a friend or business associate from the south?" A Lebanese diplomatic official asked. "How are we going to find that man to verify the information? Who knows where he's living now."

The ostensible purpose of the new regulation is to give the government some measure of control over the terrorists, radical extremists and assorted troublemakers for whom this country has long been popular. The timing of the order and the exemption for Syrians, however, were seen here as an indication of the real purpose.

Syria, which has about 20,000 troops here, is the real authority in Lebanon. The entry crackdown followed swiftly upon the revelation that "volunteers" from Iraq, Syria and Algeria were entering the country to join Palestinian guerrillas in the south.

The Syrian-dominated Arab peacekeeping force, sent here to restore order after the civil war, announced that arms and equipment would no longer be permitted into the country except for delivery to the Lebanese central government, and the entry visa requirement followed a few days later.

Syria, which is trying to defuse the explosive situation in the south and to build the authority of Lebanese President Elias Sarkis, has thus given itself the means to keep out what it considers undesirable elements, but the side effect is apparently going to be great inconvenience for the vast majority of travelers who are on innocent business.