There have been queues stretching a long as 2km (1.2 miles), as people wait to lay flowers and handwritten notes.
Indonesian worker Romlah Rosidah said she was “very surprised” with how many people turned up to pray for the victims.
“This event was only spread on social media, but [it] turned out their hearts moved,” she told news agency Reuters.
A Filipino worker said they joined the prayers “to show the Hong Kong community that we are one in this situation”.
Officials observed three minutes’ silence to begin the mourning period, and the flags of China and Hong Kong are flying at half-staff.
The fire is Hong Kong’s deadliest in more than 70 years, and the death toll has been rising each day, as officials work to recover and identify bodies.
The exact cause of the blaze has yet to be determined. Eight people have been arrested on suspicion of corruption over the renovations that had been taking place on the towers, while three others were detained on manslaughter charges.