Members of a “cynical and well-practised” grooming gang have been jailed for up to 29 years for sexually abusing vulnerable girls in Newcastle upon Tyne after plying them with drugs.

The men were convicted following an unprecedented and strongly criticised police investigation in which a convicted rapist was paid £10,000 to inform on the gang.

Newcastle crown court heard how the gang’s victims, aged between 13 and 25, were given alcohol and the class B drug mephedrone before being sexually assaulted at “sessions” in the west end of the city.

The case, which had echoes of sex grooming scandals in Rochdale, Rotherham and Oxford, formed part of Operation Sanctuary, the biggest ever investigation into child and adult exploitation in the north-east, which has identified 780 potential victims.

Sentencing the first of 14 gang members on Tuesday, the judge, Penny Moreland, said there was no direct evidence that the offending was racially motivated.

She added: “In my view, and speaking in broad terms, these defendants selected their victims not because of their race but because they were young, impressionable, naive and vulnerable.

“This is extremely serious offending against vulnerable members of our society and that is the basis upon which I intend to sentence.”

Jahanger Zaman, a 45-year-old heroin dealer, was jailed for 29 years for raping a vulnerable girl, conspiracy to incite prostitution and a string of drugs offences. Moreland said he caused “incalculable serious harm” to his victims by giving them mephedrone, known as “m-cat”, and demanded sexual favours in return.

When Zaman was interviewed by police he denied the offences, saying one of the complainants was “full of shit” and claiming the victims were being paid by the police. Roy Brown, defending Zaman, told the judge in mitigation: “He blames no one but himself.”

Zaman’s privately educated accomplice, Mohammed Azram, 35, was jailed for 12-and-a-half years for sexual assault, conspiracy to incite prostitution and drugs offences.

The court heard that Azram played a leading role in the ring, on one occasion sexually assaulting a teenage girl who was under the influence of drugs he had supplied. Azram was a drug addict with 62 previous convictions, including nine for drugs, and grievous bodily harm on a woman.

Sentencing Azram, Moreland said: “It is a common theme in this case that the young women concerned have only gradually come to realise that you and other men were not their friends and to understand that to you they were merely commodities, passed around at parties like the alcohol and drugs, available to anyone who wanted them.”

The two were jailed alongside Shafiq Aziz, 48, who was sentenced to 15 years in prison for conspiring to supply cocaine and heroin in an operation with Zaman that spanned from the Midlands to the north-east.

In victim impact statements read to the court, the women described how they had been left emotionally scarred.

One victim said she missed out on a university place as a result of the abuse. She said she was left to “feel like I was just a piece of meat” by her abusers. “I had to hear them laughing as they went about telling others what they had done to me,” she said.

“They spread all kinds of rumours about me. I got put down by them so regularly I was left with no confidence in myself.”

One victim said she did not eat for a week after suffering a come-down from mephedrone, known on Tyneside as “bubbles”, supplied to her by members of the gang.

Another woman said was mentally scarred by “these monsters who abused me”. She said: “It’s been a really long hard road. My life has been really affected by what happened to me. I have real mental health problems and don’t even feel safe in my own home. I used to be such a happy girl.”

Newcastle crown court heard that the defendants in the Operation Shelter trial, a splinter investigation falling under Operation Sanctuary, were all of “Asian extraction” and largely British-born, although some were born abroad.

John Elvidge QC, prosecuting, said the victims who gave evidence in court were all “white British”. But the ethnicity of other potential targets was not known, he said, and one vulnerable girl who did not engage with police was black, and an Asian girl was seen at a party.

One defendant, who was not sentenced on Tuesday, had made racist remarks towards a ticket inspector, but was cleared of any sexual offence.

Elvidge said eight of the gang were involved in a “well-practised and cynical” conspiracy to incite prostitution.

During the trials, the court heard how Northumbria police paid a convicted child rapist – referred to as XY – £9,680 over 21 months to find out about the times and whereabouts of parties where girls were being plied with drugs and alcohol, in order to gather evidence for their investigation.

No evidence from the informant, a British-Asian man in his 30s with links to the defendants, was put before the jury. The NSPCC condemned the use of the man, saying it raised serious questions about the force’s approach to child sexual exploitation operations. The police argued it would not have been possible to uncover the crimes in Operation Shelter using “conventional methods”.

The rest of the gang, whose offences were carried out on 15 victims between 2011 and 2014, will be sentenced later this week.