The violence in Libya was threatening to turn into a humanitarian crisis Tuesday as thousands of people fleeing into Tunisia overwhelmed relief efforts, creating a bottleneck of evacuees stranded on the Libyan side of the border.

U.N. officials moved to erect a tent city to shelter the more than 15,000 people arriving each day, largely Egyptian migrant workers but also Libyans as well as oil workers and menial laborers from Chad, Sudan and nations as distant as Bangladesh and China.

More than 75,000 people had already arrived in Tunisia since the uprising against Libyan leader Moammar Gaddafi began Feb. 17, but the sharply increasing flows over the dusty desert crossing of Ras Jdir were forcing Tunisian authorities to periodically close the border to stem the tide.

With thousands stranded on the Libyan side of the crossing, the U.N. World Food Program warned that the violence could result in a massive regional displacement of up to 2.7 million people over the coming weeks. Some have already been stuck for days trying to escape Libya, with volunteers tossing bread and water over fences to the hungry caught in the no man’s land between the two nations.

The Office of the U.N. High Commissioner for Refugees and the International Organization for Migration issued an appeal for governments to organize a “massive humanitarian evacuation.”

“The numbers are too great,” said Hafez Ben Nined, an official with the Red Crescent on the Tunisian border. “And they keep coming and coming.”

Tens of thousands have also crossed into Egypt from the opposition-occupied eastern half of Libya. But the border there, officials said, appeared less stressed given that the majority of those fleeing are Egyptians returning to their home cities and towns.

Instead, concern centered on the western border with Tunisia. The Chinese and other governments were organizing airlifts to repatriate nationals in Tunisia. After long delays, Egypt appeared to step up its effort Tuesday, deploying 39 planes to Tunisia and 24 to Libya.

But Tunisian authorities warned that the pace of the evacuation was so slow that some migrants risked being stuck for weeks in precarious conditions. Most were still in the border area or surrounding towns, many huddling together outside in night temperatures dipping below 50 degrees.

Many of the migrants were abandoned by their employers, and their cash and possessions had been confiscated by Libyan officials.

Mahmoud Mohamad, 23, an Egyptian day laborer who left Tripoli last week, said he first tried the airport there but was turned away, even after Libyan authorities demanded a cash payment from him to enter the terminal. He came out by land over the Tunisian border, but as of Tuesday evening he had already spent five days among a throng of workers and families sleeping on blankets thrown on the sandy soil.

“Where is the Egyptian ambassador? Where are my human rights?” he said. “I have been here for five nights. It’s very cold and raining. We need an airplane, a boat to take us out of here. But I don’t see anything happening.”

Melissa Fleming, spokeswoman for the Office of the U.N. High Commissioner for Refugees, said the agency had met with Egyptian and Tunisian officials to assess the crisis. She said that Egypt is sending planes to evacuate its nationals but that they can carry only about 100 people at a time.

“There are almost 1 million Egyptians in Libya, so this could go on for days,” she said. “They are pressing against the border.”

Fleming said her agency is growing increasingly alarmed at the plight of about 8,000 refugees and 3,000 political-asylum seekers, most of them from sub-Saharan African countries including Somalia and Ethiopia, who have been targeted by Libyans who suspect they are serving as Gaddafi’s foreign fighters.

“They say they are being hunted down. They fear for their lives,” Fleming said. “Some say their houses have been burnt down to the ground and they have been forced out of their homes. They are truly terrified.”

Tunisian authorities and the Red Crescent were struggling to provide food and water for those in need, and the U.N. World Food Program rushed 80 tons of high-energy biscuits to the border from Italy and moved to ship in wheat from neighboring Algeria.

“This is different from your typical refugee situation, because you have lots of people here who want to go home but not enough boats and planes to get them there,” Josette Sheeran, executive director of the World Food Program, said after a fact-finding mission at the border. “And now, it looks like the numbers are stepping up.”

The World Food Program, she said, was moving to get a shipment of food to the Libyan city of Benghazi, the center of resistance to Gaddafi’s 41-year rule, but faced severe problems trying to reach opposition-controlled cities in the west, where food and medical supplies were running short and forces loyal to Gaddafi controlled the roads.

“The stocks are emptying, and they are worrying about supplies,” she said. “We are coming up with contingency plans as we’re getting reports that [pro-government forces] are not allowing food shipments to pass into those cities.”

The relief effort on the border has been aided by an army of sympathetic volunteers in Tunisia, the nation where the uprisings sweeping the Arab world began with the forcing out of authoritarian president Zine el-Abidine Ben Ali. The Tunisian leadership remains in a state of flux after protests that have ousted two leaders over the past six weeks.

The Tunisian military has deployed troops along the border to treat the wounded and manage crowds. But the situation has become increasingly desperate, officials say, leading to calls here for an even more stepped-up international effort.

“Our problem is that we cannot absorb any more people coming in,” said Mustafa Mahzi, an activist who protested against Ben Ali and who is helping organize relief at the border. “There are now 20,000 at the crossing, 50,000 in the [border town] of Ben Gardane and more then 10,000 on the Libyan side waiting to cross. They do not have tents — they do not have anything other than their personal luggage. We need international intervention.”

Staff writer Colum Lynch in New York and special correspondent Samuel Sockol in Tunis contributed to this report.