The demonstrators came from across the south and gathered on Parade Square in the centre of Aden, waving flags of the former South Yemen and carrying banners with pro-independence slogans.
Security forces watched the crowds from a distance, particularly around public buildings and police and army posts.
Some protesters chanted slogans denouncing the national dialogue, talks aimed at drawing up a new constitution and preparing for elections, and which have faltered partly because of the southern issue.
Majed al-Shuwaibi, a member of the Movement at the rally, said the choice of date was significant.
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"The southerners are celebrating the anniversary of the October revolution... Which will continue for the re-establishment of the state," he said.
Hassan Baoum, head of the Southern Movement's supreme council, arrived from the neighbouring Hadramawt province yesterday evening and was due to address crowds in Aden, activists said.
After the former North and South Yemen united in 1990, the south broke away in 1994, triggering a brief civil war that ended with the region being overrun by northern troops.
The talks, which opened in March and were due to close on September 18, accepted the principle of a federal state but President Abdrabuh Mansur Hadi and northern delegates suggested it should comprise several entities.
But secessionists from the south are demanding a federal state made up of a north and south only.
The talks are part of a transitional process stipulated by a UN-backed initiative, brokered by neighbouring Gulf countries, which ended a year of Arab Spring-inspired protests and eased former autocratic president Ali Abdullah Saleh out of office in February 2012, after 33 years in power.