Detectives in Antwerp are searching for clues in a sewage pipe under the Belgian city’s diamond quarter after burglars apparently crawled through it to break into a bank holding safe deposit boxes full of jewels in a criminal caper fit for the big screen.

When police attended the BNP Paribas Fortis bank, they opened the still-secure main vault door to find some 30 emptied deposit boxes, a hole in the floor and a tunnel to the sewage system.

Initial investigations suggest the thieves started out in a building opposite in the early hours of Sunday and moved through a freshly dug tunnel four metres (13ft) long to connect with a sewage pipe under the road.

To get closer to the bank, it seems the burglars crawled through the sewage pipe – said to be between 40cm and a metre high – before tunnelling for a further four metres and emerging among the bank’s vaults.

The first the police knew of the heist was when the bank’s security firm sounded the alarm at 1.34pm on Sunday. When police arrived, they saw the vault was “still locked but the alarm was on”.

The bank is on the edge of a square mile that is the largest diamond district in the world, boasting an annual turnover of $54bn. It was the scene of the “heist of the century” in 2003, when more than $100m in jewels was stolen from a vault.

Willem Migom, a spokesman for Antwerp police, said: “A tunnel to the sewer system has been found. We first had measurements carried out on gas to see if it was safe to descend into it. In the sewerage we then found a second tunnel that led to a building in the Nerviërsstraat.”

In a sign of the groundwork required to pull off the burglary, a mattress and digging equipment were found in the sewage pipe.

BNP Paribas has declined to comment on the size of the burglars’ haul. Concerned customers gathered at the bank’s office on Monday as news of the break-in spread were asked by staff to disclose the contents of their deposit boxes.

“We are investigating the size of the robbery and we cannot provide further information,” the local prosecutor’s office said in a statement. “At the moment no suspects could be apprehended.”

A building opposite the bank on Nerviërsstraat was searched by police on Sunday. One unnamed person was taken in for questioning but later released.

The first the residents of the central Antwerp district knew of the incident was when police raised all the manhole covers running down the centre of Nerviërsstraat on Sunday afternoon.

One woman in an adjacent street told Het Nieuwsblad she had heard “a lot of banging” from the neighbours in the past few days.

The newspaper likened the crime to the infamous 1976 Ocean’s Eleven-style bank heist, when 13 people used the sewers to break into the vaults of the Société Générale bank in central Nice. That gang cleared more than 300 safe deposit boxes of gold ingots, cash and jewellery valued at 50m francs – about £24m today – before fleeing just in time to escape the rising sewage that was flooding the bank.

Roger Moorthamer, a spokesman for the federal police, said: “We have no idea at the moment who the perpetrators are.”

Els Liekens, of the engineering company Aquafin, told Het Laatste Nieuws the theft had been extremely risky. “I do not know how those burglars got out alive there. When it starts to rain, rainwater enters the sewers and can fill up very quickly, which can lead to drowning if you are there,” she said.