Detectives have arrested 16 people in raids across Essex in one of the biggest investigations in England and Wales into a criminal conspiracy targeting vulnerable older people.

Up to 140 pensioners, aged between 65 and 99, were allegedly conned out of more than £1m by the gang, police said.

Eighteen victims, predominantly widowed or unmarried women, died during the seven-month investigation by detectives from the Kent and Essex serious crime directorate.

Photograph: Essex police

It involved covert surveillance of the suspects. CCTV cameras revealed evidence of elderly people appearing to be accompanied to banks by some of the suspects, where allegedly they were seen withdrawing large amounts of money.

Footage also allegedly showed one victim being followed out of a doctor's surgery and seemingly befriended outside.

Detective Chief Inspector Keith Davies, the senior investigating officer, said: "This investigation centres around a suspected organised criminal gang who are believed to have become experts in the grooming of elderly people.

"The allegations are that they make friends with them, gain their confidence, visit them repeatedly and persuade them to hand over money.

"The people we have spoken to are vulnerable elderly women predominantly, who have been widowed and have no partner or family around. Some have early-onset dementia. Many are thought to have lost their life savings. Some of these people are incredibly vulnerable and it appears were clearly targeted."

The alleged victims were living in seaside towns in Essex, including Frinton-on-Sea, Walton-on-the-Naze, Holland-on-Sea and Clacton. Police believe there are more who have not yet been identified and are appealing for people to contact them if they believe they have been targeted.

Photograph: Essex police

The victims were allegedly conned into paying out cash or cheques for large sums for unnecessary building work, or work for which they were overcharged by up to tens of thousands of pounds. Others were allegedly persuaded into handing over loans of thousands of pounds.

One woman in her 90s told police she had paid £125,000, believed to be her life savings. She died last year during the investigation.

The raids took place at addresses in the Colchester and Clacton areas of Essex. The suspects, who included one woman, were taken to police stations to be questioned on suspicion of fraud and money laundering. Among those arrested were two brothers and one of their sons.

Assistant Chief Constable Gary Beautridge, head of the Kent and Essex serious crime directorate, said the operation was one of the biggest of its kind in the country.

"This involved covert activity, intelligence, financial investigations, technical experts and specialist support for victims," he said.

The victims' vulnerabilities led to their being interviewed with methods similar to those used for questioning child victims. They have been referred to support services as the inquiry continues.

Those arrested were taken into custody at police stations across Essex. They are to be be questioned over a range of offences, including money laundering and fraud.