Below is an article published by the Wall Street Journal:

Hundreds of thousands of refugees from Pakistan's devastating floods are seeking shelter in this city of 18 million, exacerbating ethnic strife that has already escalated this year and threatens to destabilize the government of President Asif Ali Zardari.

Most of the refugees are ethnic Sindhis from areas outside Karachi, the capital of Sindh province, whose homes and livelihoods have been destroyed in the flooding that began more than three weeks ago.

The United Nations says 800,000 people are stranded by the flooding, which has severed major roads in Sindh and nearby Baluchistan province. Some 1,500 people have been killed and six million made homeless by the deluge, which started in the north but swept south along the Indus River and continues to threaten to submerge towns and villages in Sindh and Baluchistan.

Sindh's provincial government has set up camps on the outskirts of Karachi, where 30,000 people are trying to keep their families together under tarpaulins in the searing heat.

The local administration says it is expecting and can cope with up to one million of the refugees. But many others who have made it to Karachi say they are being turned away from shelters on the outskirts and are pouring in to the city, deepening ethnic rivalries with Karachi's majority ethnic community that have simmered for years.

Police and paramilitary forces opened fire Monday on hundreds of mainly Sindhi refugees who, aided by a Sindh nationalist party, had begun squatting in 200 unoccupied, private apartments in a northern suburb of the city. Three people were killed, including a young boy, and 16 injured when the police fired live ammunition and tear gas into the crowd to disperse them after some refugees began throwing stones, eyewitnesses and local government officials said.

On Wednesday, police raided the house of a Sindh nationalist politician, Safdar Sarki, who organized the refugees' settling in the apartments without permission from the builder, one of the largest in Karachi. Mr. Sarki, in an interview, said he had sent the refugees there.

For Karachi's dominant group, the Urdu-speaking Muhajirs, the influx of Sindhi refugees poses a threat to the established order.

"If they come in hundreds of thousands, how will they survive?" says Khawaja Izhar ul Hassan, a member of the provincial assembly from the Muttahida Qaumi Movement, or MQM, which is largely a Muhajir political party and forms part of Sindh province's ruling coalition government.

The refugee crisis is adding a new dimension to a running struggle for control of Karachi between the MQM and ethnic Pashtuns, a group with origins in northwest Pakistan. The number of Pashtuns in Karachi has swollen in the past few years as many have fled fighting between the Pakistan Taliban and the military in their homelands bordering Afghanistan.

Almost 1,000 people have died in Karachi since the start of the year, many in violence largely between Muhajir and Pashtun armed groups. Mr. Zardari's administration has been unable to stem the violence.

The MQM has cast its opposition to the wave of Pashtun migration as a necessary push-back against the Taliban's growing influence in Karachi. As evidence, officials point to a number of arrests in the city, including the detention in February of the Taliban's operations chief, Abdul Ghani Baradar.

The wave of Sindhi flood migrants may be harder to counter. Sindhis are Karachi's indigenous ethnic group. The Muhajirs, Muslims of various ethnicities from India, arrived in the city after the 1947 partition of the subcontinent.

Sindh nationalist parties battled the MQM in the late 1980s over control of land in Karachi but in recent years have largely stayed out of the violence. While the MQM has joined in coalition with Mr. Zardari's Pakistan People's Party—in both the Sindh provincial and national governments—Sindh nationalist politicians largely boycotted elections and pushed for greater rights for the Sindhi community.

Ethnic Sindhis number about four million people, smaller than the Muhajir community. But Sindhis dominate in rural areas of the province. Mr. Zardari is an ethnic Sindhi and the PPP draws large votes from farming communities that have been affected by the floods.

The provincial government has set up 30 camps around Karachi. In the largest, in the northern suburb of Gulshan-e-Maymar, some 10,000 refugees are sheltering, 10 to a room on average, in unused government-owned apartments. Outside, local volunteers have started primary school classes for 450 children. The government has set up a water filtration plant and hooked up the apartments to a generator.

At another camp, in Bin Qasim Town to the east of the city, thousands of refugees lie under makeshift tents in the searing heat. They receive water from large trucks and two meals a day. On the outskirts of the camp, newly arrived refugees carrying children with distended bellies shelter beneath plastic bags strung together over branches.

Deen Mohammad, a 30-year-old farmer from Jacobabad, a town in northern Sindh, lost his house and goats when the Indus River burst its banks. His sister is still missing, he says.

"Nothing is left behind," said Mr. Mohammad, who made his way to Karachi with his wife and four children on a train service provided by the government. "There's no livelihood there. We'd prefer to stay here."

Others have found refuge where they can. Mr. Sarki, president of Jeay Sindh Tehrik, a Sindh nationalist party, sent his cadres to the train station to direct refugees to the unoccupied apartments in a Sindh area of the city.

They were meant to be there temporarily, he said. But the police action has shown the city government, which has been dominated by MQM for 25 years, is worried about an influx of Sindhis changing the balance of power in Karachi, Mr. Sarki said. "If one million Sindhis came here, the demography would be changed," he said. "These people are trying to get rid of Sindhis and dominate Karachi to show that Karachi belongs to them. It's totally immoral."

At the site of the killings, bullet marks pocked the walls and blood was smeared over the ground. A former makeshift medical clinic lay destroyed, with hundreds of pills trampled into the ground. Even after the violence, a few refugees continued to live in the flats.

"Where can we go?" asked Mai Dini, a middle-age refugee from Jacobabad, as others piled their meager belongings on colorfully decorated trucks.

In a nearby hospital, Sohrab Sarki, a laborer from a bakery in Jacobabad, recovered from a bullet that passed through his left side but missed his major organs. He said the incident was an attack on Sindhis. "They don't want others to come," he said.

Mr. Hassan, the MQM politician, says the police should have acted with more restraint. The MQM, he said, welcomes refugees on a temporary basis and if they are settled in official government camps or relatives' houses.

The Sindh nationalist parties aren't contributing to the relief efforts and are using the refugees to further their political agenda, Mr. Hassan said. "They are looking for places to capture. We will not allow them to use private property in the name of refugees."