The densely-packed crowds are demanding that the bill – which would allow extradition from Hong Kong to mainland China – be completely scrapped.
The region’s leader, Carrie Lam, on Sunday apologised for causing “disputes in society” over the bill.
A day earlier she delayed the bill plans after mass protests this week.
Many protesters, who fear increased Chinese influence over Hong Kong, are calling on Ms Lam to resign over the unrest.
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The protests have so far remained peaceful, in contrast to the previous major demonstration on Wednesday which saw clashes between protesters and police that injured dozens.
By early Sunday afternoon, large crowds had gathered in the city’s Victoria Square, many wearing black or carrying white flowers. Organisers said they hoped more than a million people would attend, though no formal estimates of the crowd have yet been made.
