Some have changed their appearance, shaved their heads and beards, taken to wearing glasses or appearing on the street in three-piece suits instead of blue jeans and carrying briefcases instead of knapsacks.

Others move from place to place, never sleeping in the same bed twice, yet emerging to hold clandestine meetings or even, in one instance, to taunt the authorities by conducting a press conference.

Since the South African government imposed a nationwide state of emergency last week and began rounding up hundreds of antiapartheid activists, hundreds and perhaps thousands of others have sought to elude the net.

This story was written under the press restrictions imposed with the government's state of emergency, and some references have been omitted.

Some activists have simply disappeared, seeking anonymity in downtown apartments or in South Africa's remote rural homelands. But many others remain in a nether world, neither above ground nor below, maintaining contact with other activists and the groups they lead while keeping one eye on the front door.

They call it "being on the air," said Seth Mazibuko, a Soweto activist in the United Democratic Front, the country's largest antiapartheid coalition. He has been staying away from his job and spending his nights in an undisclosed location in downtown Johannesburg since the emergency began.

"We don't want to run amok in public but we don't want to disappear either," he said in an interview today. "The UDF believes that the organization is people and we must always have a way of communicating with people. We must be where our eyes can see the sheep."

He estimates that thousands of the front's leaders and members are in hiding. Included are the two dozen or more members of the front's national executive committee and hundreds of officials in the front's various regional executives.

"It's really like a morgue here," said Audrey Coleman, a member of the Detainees' Parents Support Committee, a human rights group that maintains an office in Khotso House, a nerve center for religious and political activists in central Johannesburg. "All of the activists who work in these offices are in hiding."

Among those who have vanished in recent days are trade union leaders such as Cyril Ramaphosa, general secretary of the National Union of Mineworkers, and political activists such as Murphy Morobe, Mkhuseli Jack and Henry Fazzie, all of them nationally known officials of the United Democratic Front, South Africa's largest coalition of antiapartheid groups.

While they cannot be located, all of these people have managed to stay in touch in recent days. Ramaphosa got a message through to union president James Motlatsi telling him to postpone contract negotiations scheduled for yesterday with the South African Chamber of Mines. Informed sources said Ramaphosa also suggested in the message that Motlatsi ought to "disappear" as well.

Fazzie, the UDF president in the Eastern Cape region who some say is partial to disguises, has put in appearances at front meetings in Johannesburg and Cape Town in recent days, sources said, and has even flown on the state-run airline.

Jack, a flamboyant boycott organizer also from the Eastern Cape, managed to hold a press conference last Friday in a Port Elizabeth township before disappearing again.

Unlike last year's partial state of emergency, which took many activists by surprise, the new crackdown was expected. Government officials inadvertently telegraphed the move in advance in their push to get stringent new security legislation through Parliament before June 16, the 10th anniversary of the Soweto uprising.

In parliamentary debates, they warned that they would have to resort to tough alternative measures to quell illegal demonstrations and anticipated violence if the bills were not enacted in time. When they were not, many saw the emergency as inevitable.

"Last time many people were taken completely by surprise and they were either picked up or they disappeared into holes and didn't communicate with each other," said Mark Swilling, a political science lecturer at the University of Witswatersrand here who specializes in black politics. "They got depressed and their organizations were quite severely crippled.

"This time they've had warning and they've had the experience, so that an efficient communications network exists. Most of the main brains have not been picked up."

The most efficient network, Swilling said, is the various street committees that the UDF has set up over the past year in major black townships like Soweto and those outside Port Elizabeth. The groups are purposely kept small -- from 4 to 10 people -- so that the meetings remain secret and do not draw official attention.

The street committees have become the base of a large pyramid of interlocking yet self-contained cells. Each committee provides a member to an area committee, which in turn feeds the front's regional executive. Information spreads rapidly, yet few members know the identities of anyone outside their own committee.

UDF officials stress that these committees are not illegal and contend they are not linked to the outlawed African National Congress. But the government clearly considers them part of the ANC's network and police officials have charged that the UDF is working with the ANC in seeking to violently overthrow the state.

Street committee meetings have been held almost nightly since the emergency was imposed and UDF leaders have been in attendance. They have also found ways of meeting with each other. For a while last week, sources said, some leaders met regularly in a minivan painted like a taxi as it drove the streets of the townships here.

Similarly, Dali Mpofu, president of the Black Student Society at Witswatersrand, has shown up on campus virtually every day for brief meetings, according to friends, then vanished again. He is among about 200 to 300 Witswatersrand students believed to be in "hibernation," another local term for the underground.

The experiences of Mazibuko, a leader of the Orlando Civic Association, a UDF affiliate, are not atypical of many front officials. He was one of the student leaders of the Soweto uprising 10 years ago when police opened fire on protesting students touching off nearly a year of civil unrest that presaged the last two years of racial violence here.

He spent 10 months in detention, followed by a five-year term on Robben Island, a maximum security prison, and has lately worked as a researcher and field worker for the South African Council of Churches. When police crackdowns begin, Mazibuko tends to disappear.

His beard has gotten shorter and his clothes considerably neater. "You try to look like someone who works as an executive at Anglo-American," the country's largest conglomerate, he said. "You try to look like a person who cannot be arrested."

There are other rules to the game, he said, including: don't get excited around the police; don't go too far "overboard," meaning above ground; but also, be sure to communicate with those who can communicate to others that you are still in circulation.

The need to remain in circulation compelled Mazibuko to risk a visit to Soweto on Monday, the 10th anniversary of the uprising. He briefly visited his home, then spoke at a clandestine political rally. Daniel Motsitsi, a UDF youth leader, also attended the meeting.

Minutes after he left the house, Mazibuko says, armed men arrived looking for him. Because of the South Africa's emergency restrictions on press coverage of police activities, it is not possible to reveal anything more about the incident.

Life underground has a code-language of its own, Mazibuko said. When people say, "We're going to play hockey," that means a clandestine meeting is planned and the name of a hockey field is used as code for the location. The Zulu word umkgabulo, which means "to sip from a calabash," is used to assign someone to research a topic for the meeting.

But there is little romanticism in the bleak existence of being on the run. "My education is in tatters, my family has been dismantled, my only child doesn't know about fatherly love," said Mazibuko.

He said he sent off his 8-year-old daughter to live with his father-in-law long ago in case of attacks on his Soweto home and he has not seen her for months. Recently she sent him a card. The message read: "Be of good cheer."

"I've been doing this kind of thing for more than 10 years and I'm definitely breaking," he said.

Audrey Coleman of the detainees' parents group said youngsters who are afraid to go home because they believe the police are looking for them often stop by her office seeking help. She can give a legal referral and some words of comfort, but often little else. "It's a heartbreaking, dreadful thing," she said.

She and her husband Max have a special reason for concern about those who have disappeared. Their three sons -- Neal, 28, Keith, 25, and Colin 23 -- are all political activists and have all stayed away from their homes since last Thursday, although Audrey Coleman will not say that they are underground.

"I don't know where they are and I never ask," she said. "As long as they're safe, I'm happy."