By Ashwin Manikandan, Arpan Chaturvedi and Munsif Vengattil

BENGALURU/NEW DELHI (Reuters) -The insolvency of Indian education technology company Byju's threatens to be the biggest upset in a celebrated startup sector, unleashing a long battle by thousands of panic-stricken employees to recover dues and protect their careers.

Once a darling of global investors, valued at $22 billion in 2022, Byju's became popular by offering online training courses during the COVID-19 pandemic, but is now locked in a dispute with U.S. lenders seeking $1 billion in unpaid dues.

Reuters interviews with dozens of its employees, parents of students and a review of WhatsApp chats show a growing sense of desperation as people plan to take the offensive against the company whose board stands suspended with assets frozen.

"A lot of people, including myself, have stopped taking classes because there's no point doing charity for the company anymore," Sukirti Mishra, 29, said on a conference call with Reuters held by about 60 employees of Byju's unit WhiteHat Jr.

Mishra, who once earned $1,200 a month teaching mathematics courses, now faces the anger of parents after she halted classes for their children, she said on the call attended by Reuters.

But she herself is struggling to pay medical bills and loans installments, after having gone unpaid for months.

Byju's, which is battling the insolvency move in court to try and regain control, did not respond to Reuters' queries. In court papers it has warned of a total shutdown of services as the insolvency proceeds.

India's Supreme Court on Thursday declined to agree to Byju's request to halt the insolvency process which is ongoing due to the legal protest by the company's U.S. lenders.

After three months without pay, many of Byju's 27,000 employees are considering street protests or lawsuits.

Since a court-appointed officer took charge of the company, about 3,000 of them have filed claims, providing bank statements as proof, said one senior company executive, who spoke on condition of anonymity.

"I guarantee this: When we regain control, your salaries will be paid promptly," company founder and former billionaire Byju Raveendran assured staff this week in an internal memo seen by Reuters.

Employees face a long road ahead as it could take months to find a new buyer, or liquidate Byju's assets. And the law gives no guarantee that employees and teachers can recover all their dues in the end.

Byju's, started in 2011, has suffered numerous setbacks in recent months, from boardroom exits and criticism over delayed financial disclosures to an auditor resignation.