JOHANNESBURG , South Africa ( CNN ) -- South African police and military units raided three hostels Thursday in a clampdown on xenophobic attacks that have left more than 40 dead , authorities said . Twenty-eight people were arrested in the `` extremely successful '' first joint operation , according to a statement from the South African police and South African National Defence Force . The statement said 150 kilograms ( 330.6 pounds ) of marijuana was seized along with firearms , ammunition and suspected stolen property . Two of the hostels raided were in Jeppe and the third was in Cleveland , all in the Johannesburg area . South African President Thabo Mbeki approved the deployment of the army Wednesday to help stop the attacks , which have drawn condemnation from South African officials and other African leaders . Police said 42 people have been killed in the violence , which began last week in Johannesburg 's Alexandra Township and has been concentrated in the city 's poorest areas . Some 28,000 people have been displaced by the violence , Hangwani Malaudzi , a spokesman for the Ministry of Safety and Security said . And more than 400 have been arrested for crimes ranging from murder , to causing a public disturbance , he said . The country has also seen a disturbing throwback to the 1980s apartheid-era lynching tactic of `` necklacing , '' which was widely used in the townships at the time . Used on suspected informants , the `` necklace '' is a car tire , filled with petrol , put around the person 's neck and set alight . The victims are mainly immigrants and refugees from other parts of Africa , including Zimbabwe , where a devastated economy has sent at least two million people across the border in search of a better life . Some say the attacks stem from a long-standing feeling among locals that the number of immigrants in South Africa results in shortages of jobs and essential needs . Inadequate housing , a lack of running water and electricity , the rising prices of food , and escalating crime -- nearly 20,000 people were slain in South Africa last year -- add to the resentment . Watch analysis of reasons behind xenophobic attacks » . South Africa 's army has been patrolling riot-hit townships to restore calm as the country counts the cost of the xenophobic attacks . Brigadier General Kwena Mangope of the South African Department of Defense said troops were assisting the South African Police Service , the national police . `` We are in a supporting role , '' Mangope said . `` We are not taking over the operation . '' He said the army was providing troops and equipment , such as helicopters and tents . The deployment of the military and the continued operations of the police service helped quell the violence , Malaudzi said . Community groups , churches , and communication between police and affected communities also helped the situation , he said . `` The message is going out there to say we condemn this kind of action , and ( to ) those that are involved in instigating this violence , the law is going to come down on them very , very seriously , '' Malaudzi said . According to South Africa 's latest census , the country has about 45 million people . The South African Department of Human Affairs estimates that more than 4 million people reside in South Africa illegally , but that figure is based on a 10-year-old study and some feel the number of immigrants in South Africa is much higher . Some say that millions have recently fled to South Africa from Zimbabwe because of violence there since the county 's stalled election . A presidential run-off between long-time leader President Robert Mugabe and his opposition challenger , Morgan Tsvangirai , is scheduled for June 27 in Zimbabwe -- three months after the initial vote . Zimbabwe also is in the midst of an economic collapse , with nearly 80 percent unemployment .
Army patrol street to restore calm after violence targeting foreigners flared . NEW : Police , military in first joint operation raid three hostels . Attacks have been concentrated in Johannesburg 's poorest areas . Zimbabweans who have fled their own country are driven from squatter camps .