According to the UK-based war monitor, the Syrian Observatory for Human Rights (SOHR), anti-Assad fighters entered the city of Homs and "took control of several neighbourhoods".
Rebel commander Hasan Abdul Ghani announced the "complete liberation" of Homs and wrote on X that more than 3,500 inmates had been freed from prison.
The fall of the city to rebel forces would be another major blow to Syrian President Bashar al-Assad as it would isolate Damascus, cutting the city off from the coastal heartland of his family's minority Alawite sect.
It would also be a symbolic victory, as Homs served as the opposition's stronghold in the early days of the civil war, which broke out in 2011.