Rioters attacked government and ruling party buildings as security forces killed one demonstrator and injured four other protesters Wednesday in the forth day of rioting that has engulfed the country's south.

The death was the first in the four-day violence, spurred by unemployed youths and retired officers angered at unfulfilled promises of jobs in the military. Earlier, 15 other protesters were hurt in the riots.

The military has deployed tanks and armored vehicles since Tuesday to several southern cities, which are the stronghold of the opposition Yemeni Socialist party that is behind the thousands of southerner former army officers, political activists, and jobless young men protesting against what they perceive as unequal treatment at the hands of northern authorities.

Once divided, North and South Yemen were united in 1990 under northern President Ali Abdullah Saleh. In 1994, the south tried to secede but was defeated by the north in a civil war that lasted several months.

Troops from the southern army fled into the mountainous hinterlands and into Saudi Arabia, where they hid for years, returning only after the government issued an amnesty and promised to readmit them into the army.

Southerners have also complained that they are kept out of government jobs — a main source of employment in the south — in favor of northerners brought in to fill the bureaucracy and security forces.

On Wednesday, about 1,500 demonstrators attacked a government compound and set fire to the headquarters of the ruling National Congress Party in the city of Tora al-Baha, some 100 kilometers (60 miles) north of port city of Aden, according to eyewitnesses.

Meanwhile, in the province of Dhalae, 220 kilometers (135 miles) south of the capital San'a, some 5,000 protesters hurled stones at security forces which responded by shooting into the air and firing tear gas, a security official said. He spoke on condition of anonymity because he wasn't authorized to speak to the press.

The ruling party spokesman, Tarek al-Shami, described the riots as an "act of sabotage that targets national unity, incites hatred and sectarian tensions." Speaking to reporters in San'a, al-Shami said those behind the riots were a "minority that serves no one but its own interest."

More than 120 protesters were detained and troops sealed off main streets in southern cities. Government institutions, shops, restaurants and schools closed their doors, as people stayed at home, eyewitnesses also said.

After detentions Wednesday of their activists, opposition parties held a joint sit-in-protest in San'a, calling on the government to release those detained, end its prejudice, enlist former officers in the military and provide job opportunities to the people of the south.

Socialist Party Secretary-General Yassin Noman accused the government of cracking down on peaceful calls for reforms and democracy.

"Arrests are aimed at terrorizing activists," he said at a conference that was attended also by the Islamic opposition Islah Party.

The poorest country in the Arab world, Yemen is home to heavily armed tribes that barely acknowledge the authority of the central government as well as a persistent al-Qaida presence that has attacked and killed foreigners on several occasions.