At 10:30 a.m. yesterday, when President Carter welcomed British Prime Minister James Callaghan with flags, brass bands, massed troops and all the trappings of state, the guns on the White House lawn were jsut for ceremony.

But two blocks away at the police blackades on the empty streets around the District Building, the guns were for real.

Washington was going about its business on the second day of terrorism - a day when most appearances said business as usual. But behind the facade of crowded downtwon streets and busy stores and appointments being kept, lay the tensions and ambiguities of fear.

Callaghan got no 10-gun salute at the White House: it was canceled for fear of startling some edgy Hanani Muslim holding hostages just a gun-shot away.

Most obviously affected was the Jesish community in and around Washington. Synagogues were closed and locked, nursery schools canceled, fears voiced.

Officials of 15 national Jewish organizations met with representatives of the Jewish Community Council of Greater Washington yesterday morning.

"Our paranoia is not justified . . . yet," said Rabbi Stanley Rabinowitz of Adas Israel Congregation at Connecticut and Porter Streets NW."I say 'yet' because I have a sinking feeling that there is more brutality in the B'nal B'rith building than elsewhere.

"I try to calm people by telling them that we're not the one one. There are two other buildings as well. But I must confess I have to keep saying that to myself."

Aside from rush hours, traffic moved better than it did the day before, and people walked the sidewalks just blocks from the three hostage sites with apparent unconcern.

In Rock Creek Park, cyclists and joggers moved regularly along the bike path below the towers of the Islamic Center at Massachusetts Avenue and Waterside Drive where nine hostages were being held.

"I hadn't though about the mosque being so close, but I don't think there's any real danger here," said Tom Lally, a 26-year-old restaurant manager out for a midafternoon run.

"I'm concerned about the hostages but I don't feel like it really affects me."

Just one mile up the parkway at the National Zoo, information officer Billie Hamlet reported things "fairly busy" with visitation normal for early March.

But she said she had several calls from prospective visitors asking how close the mosque was and whether any trouble was expected at the zoo.

"One woman said 'They can't shoot a mile, can they?'," Hamlet said. "And we had one school group from Falls Church cancel a visit. They said the principal had forbidden any trips into Washington."

At the National Gallery of Art, the wait to see the popular King Tut exhibit shrank to two hours, the shortest in weeks, and buses from out of town were reported far fewer than normal.

"Something like this stirs up a lot of general anxiety, even among people not obviously affected," said Dr. Norman Tamarkin, a Georgetown psychiatrist. "Tempers are shorter . . . it's a reminder of our vulnerability . . .

"If you look around in restaurants, you may see as many people as before. But they're not laughing as they usually do. At a time like this, frivolity recedes."

Last night's performance of the play "Bully," at the National Theater directly across from the District Building was canceled.

The National Geogrpahic Society told its nearly 600 employees not ot show up for work after police advised them to keep the society's headquarters on 17th Street closed because it is near the B'nai B'rith headquarters at 17th Street and Rhode Island Avenue. A few Geographic employees defied the ban and showed up for work anyway.

Some business strengthened security. WRC-TV, for example, doubled the number of guards at its studios and posted guards at its adjacent transmitting tower.

A number of protection agencies reported an upswing in injuiries, and virtually all said they expected terrorism to be good for business.

"A number of my permanent clients are calling, wanting assurances that if something like (the) 1968 (riots) happens again they can be protected," said Frank Godby, owner of Arlington Protective Agency. "But they're not going to spend any more money until something breaks their windows."

In some places people were worried about just that. Sam Roberts, director of the Hewbrew Home for the Aged in Rockville, said residents at the home were "very upset" about the terrorism.

Part of the concern, he said, was due to a firebomb attack on the home in 1973. No one was injured in that incident, he said, but the memory remains.

Dr. Arnold Meyersburg, a Kensington psychiatrist with many elderly Jewsih patients, said he thinks the full psychological impact of the terrorism on many Jews "hasn't really hit yet."

Eventually, he said, those Jews who haven't already done so will be looking at the terrorism reports in newspapers and television and, regardless of how many non-Jews are involved as victims, will say "They're coming for us again."

"The repetition of this sort of thing in Jewish history is quite overwhelming," he said, "even though centuries may go by with very little persecution . . .

"Those most deepy engrossed in their religion are the most mindful of it . . . the burden of experience . . .says prepare first and then relax the preparations later if you don't need them."

Jews, he said, are inclined to see terrorism only as a destructive force, and themselves as the first victims of any such force. Yom Kippur itself, the holiest day of the Jewish faith, commemorates the martyrdom of those victims through the ages, Meyersburg said.

Jews weren't the only ones afraid.

Yesterday, among the solid brick homes of upper 16th Street Northwest, near the Hanafi headquarters at 16th and Juniper Streets, several neighbors said they were terrified by the events downtown and too frightened to speak about the Hanafis at all.

Others, however, said the Hanafis had been largely a peaceful if mysterious presence in the neighborhood, emerging briefly from behind locked doors and barred windows and rarely speaking to their neighbors.

And the business of the neighborhood went on. While a Hanafi guard paced before the Tudor headquarters armed with a machete and a Japanese sword yesterday, Ted Rice, 59, Jim Guerrini, 22, and Eric Clarke, 23, worked unconcernedly two doors away surveying a neighboring back yard for a swimming pool installation.

"We asked him (the guard) if this was Juniper Street and he swung that sword at me and said to go away," Clarke said. "He had the sword, so I went. But we aren't worrying about it.We got work to do."

Many others in Washington, apparently, felt the same. Wayne Kennedy, director of the Washington Area Convention and Touists Association said the terrorists don't seem to be hurting tourism so far. A check of Washington hotels turned up few cancellations.

Ed MacMillan of the International Inn said he might know more this weekend when two business conventions are scheduled, but right now business is up: his hotel is only a few blocks from B'nai B'rith headquarters, he said, and television crews are spending the night.