Demonstrators briefly blocked the road linking the capital Sanaa to the southern port of Aden with burning tyres on Tuesday, after riot police had dispersed a crowd by firing into the air.
The protesters complained that a number of youths from the region had not been admitted into the army after responding to a recruitment campaign.
There were also protests in the towns of Dhaleh and Habilayn on Sunday and Monday.
In Dhaleh, rioters set fire to at least two police stations, burnt military vehicles and tried to storm the state-owned bank, an official told the Associated Press news agency, adding that at least nine demonstrators were injured.
Several protests have been held in south Yemen in past months to demand greater state assistance for more than 60,000 retirees from military and civil service, most of whom insist they were forced into early retirement.
Last week, some 20,000 demonstrators took to the streets of Dhaleh, in response to a call from the Retired Army Association of southern veterans.
Residents of southern Yemen often complain of discrimination since a 1994 secession bid led by socialists was crushed by northern forces loyal to Ali Abdullah Saleh, the president.
Officers and soldiers from the southern army fled into the mountainous hinterlands and into Saudi Arabia for years, returning only after the government issued an amnesty and promised to readmit them to the army - a promise that the southerners say has not been kept.