Leonel Poveda Sediles, carrying a Soviet-made submachine gun and wearing well-tailored army fatigues, jumps onto the bus as it is about leave this outpost at the Costa Rican border.

"We want to inform you that where you are going the dictator has left us only ashes," Poveda tells those on board, who, along with an estimated 100,000 other refugees, are gradually returning to their homeland after waiting out Nicaragua's bloody civil war in neighboring Costa Rica.

"The Bank of Nicaragua is empty. Work and service is what awaits you," Poveda continues, giving the bad news first. "But you are the good people. You should participate. Do not permit a repetition of the dictatorship."

Poveda, a high-ranking officer in the Sandinista army whose nom de guerre is Comandante Comanche, makes his little speech 40 or 50 times a day, personally addressing each group of returning refugees as they pass through Penas Blancas for the interior.

The refugees, many of whom do not yet know whether their homes are still standing or if their families are still alive, are both frightened and excited as they begin the last leg of their uncertain journey home.

Poveda's speech is designed to warn - and calm - them. It obviously succeeds: each time he finishes, he receives a hearty ovation before he sends the refugees on their way.

But the speech has another, more political purpose than just comforting the 1,000 or more refugees who have been passing through here each day for the past two weeks.

The Sandinistas know that, despite their current popularity and the deep hatred most Nicaraguans felt toward Anastasio Somoza and his brutal, corrupt and now defeated National Guard, there is mistrust of the revolutionaries who are now in control.

While not betraying their hopes and ideals, the Sandinistas are trying to dispel their image as wild leftists bent on turning Nicaragua into a second Cuba - which would for many Nicaraguans simply mean substituting one form of repression for another.

"I want them to feel they are on free territory, that they are entering another atmosphere," Poveda explains after the bus leaves. "We're educating the people. To explain what we expect from them and what they can expect from us."

To demonstrate to the returning refugees that Nicaragua has changed since Somoza's ouster, Poveda also makes them wait their turn in line. "It makes some of them angry," he says, "because in Somoza's time people brought a letter from someone in the National Guard or the government and skipped over the others who were waiting.

"We have changed that. There will be no favoritism in the new Nicaragua."

The scene at the border, at least at Penas Blancas, the principal cross-point on the Pan American Highway between Managua and San Jose, is one of controlled chaos.

Long lines form as the refugees first wait for the border station in Costa Rica to open at 8 a.m., then wait for the Sandinista guards to check their cars or their belongings, then wait again for Poveda to approve personally every entrance permit that is granted.

So far, the traffic has been almost entirely one way. A sign over the door to Poveda's office puts the government's policy succinctly: "Strictly prohibited. Only persons with authorization from the Ministry of the Interior can leave the country."

Although the new government has promised to set up an office to process exit applications for thousands of Somoza supporters who want to leave, there is little sign at Penas Blancas that many exit permits have been approved yet.

Poveda and the new government are determined to detain any former National Guard soldiers trying to enter or leave Nicaragua, a touchy subject that Poveda tried to dodge when two American journalists visited.

Off to the side of the central immigration office is a small building with barbed wire and Sandinista guards around it. Inside the building, a dozen men, obviously being held prisoner, were staring at the refugees as they carried their belongings through customs and loaded them onto the waiting buses.

Asked about the prisoners, Poveda said they were men who had been detained only that day because they were hanging around the border station for no apparent reason.

But, later, a young Sandinista who was standing guard in front of the building said most of the prisoners had been there several days and some of them for as long as a week. "They are under investigation," the young guard said. "Some have admitted being members of the Guard."

The refugees comprise a cross section of Nicaraguan society, ranging from children sent by their parents to Costa Rica to older people, ranging from wealthy businessmen reutrning in Mercedes Benzes to peasants returning on foot. They do not seem to notice the prisoners.

Each of them has a horror story of why he or she finally decided to escape to Costa Rica and most of the stories begin and end with fears that the National Guard would bomb their houses or take their young sons to fight, untrained, against the Sandinistas.

They are returning now because they believe their country is finally at peace. They do not know exactly what kind of political and economic system the Sandinistas will create now that they are in power.

But they are watching intently, how the Sandinistas behave. Poveda is well auare of this and is determined that the initial impression be a good one.

He even apologizes to each busload of refugees for the hours they have spent waiting to return to their country. Things have, however, improved.

"The first bus, two weeks ago, took three days to cross," he says with a smile. "It has only taken you three hours."