The standoff began after the four men, reportedly armed with grenades and rifles, walked into the bank and demanded money.
However, they were interrupted by an off-duty police officer who went to use a cash machine in the bank, AP reported.
It was not immediately clear how many hostages were freed, as hundreds of police, paramedics, relatives and onlookers gathered outside the bank.
Initial reports had said that at least 30 hostages, including several children and a pregnant woman, were being detained, many of whom could be seen pleading for food and water through the bank's windows during the incident.
"We are in real danger. They can kill us. Please help," read a placard placed at one of the branch's windows.
The hostage takers had already freed two elderly people and another pregnant woman on Monday shortly after the crisis began, police told AFP, while more managed to flee.