The demonstrations are the latest in a steady series of protests there underlining the lingering tensions between the north and south more than a decade after a 1994 southern attempt to secede was quashed.
The south, which was a separate country until it unified with the north in 1990, complains that it is discriminated against by the northerner-dominated government.
The latest unrest also comes as fighting has erupted in the north between government forces and Shiites rebelling over neglect and discrimination. The many conflicts have raised concern over the strategically located country's stability.
Nasser al-Khubbogi, a lawmaker from the south, estimated that at least 100,000 people were protesting in across southern Yemen, calling on the visiting secretary general of the Arab League, Amr Moussa, to "liberate the south" from the north.
The Yemeni government did not comment on the demonstrations or provide crowd estimates of its own.
The southerners are demanding the Arab League oversee talks with the government that would allow the south to secede, said al-Khubbogi.
Moussa told reporters after meeting with Yemen's president that the country's leadership seemed open to any ideas preserving the country's unity.
"I have sensed from the Yemeni President Ali Abdullah Saleh, sincerity for dialogue with all the political factions inside and outside the country, on condition on keeping Yemen's unity and stability," Moussa added.
Since last August, several southern cities and towns have been the scene of constant demonstrations by disaffected southerners.
Yemen, the Arab world's poorest country, is beset on several fronts: the Shiite rebels in the north, secessionists in the south, al-Qaida militants in the hinterlands and pirates off its coasts and disgruntled tribes all over.
Yemen is also Osama bin Laden's ancestral homeland, in whose waters al-Qaida bombed the USS Cole in 2000, killing 17 American sailors. More recent attacks include an assault on the U.S. Embassy a year ago.