The Syrian Observatory for Human Rights said on Thursday that the number of people leaving a rebel pocket in eastern Ghouta for government territory had risen to 12,500.
The mass departure comes amid a drive by government forces to recapture the region.
According to the UK-based war monitor, it is the biggest group of people to flee the rebel enclave since government forces launched a deadly offensive last month to recapture it.
A pro-regime TV station showed the exodus towards government-held positions on the outskirts of Hammouriyeh town,
What had been the Syrian opposition's largest piece of territory near the capital Damascus has now been split into three encircled pockets by the government offensive.
Despite air strikes and shelling, an aid convoy managed to enter the enclave on Thursday where hundreds of thousands of people remain trapped - as the Syrian war enters its eighth year.
United Nations Secretary-General Antonio Guterres last month said what was happening in Eastern Ghouta was "hell on earth".