Authorities in Turkey detained 157 people, including opposition party members and a former mayor of Izmir, early on Tuesday, local media and the opposition Republican People's Party (CHP) said.

The Izmir prosecutor ordered the detentions in the early hours as part of an investigation into corruption, tender rigging and fraud in the west-coast city, broadcaster NTV reported.

The coastal city is seen as a bastion of Turkey's political opposition.

Izmir's former mayor, Tunc Soyer, and numerous "senior officials" were among those detained in Izmir, Murat Bakan, the vice president of the CHP posted on X.

"We are faced with a process similar to that in Istanbul," Bakan said, referring to the March 19 arrest of Mayor Ekrem Imamoglu.

Bakan added that Senol Aslanoglu, the party's regional president, was among those detained on Tuesday.

"These dawn arrests were not a legal obligation, but a clear political choice," Bakan added, saying that many of those detained had already been under investigation. "If they had been called to testify, they would have done so."

"This dawn detention is not a legal necessity but a clear political choice. Indeed, President Erdogan signaled these operations from the palace a few days ago. The scene we are witnessing today is not a product of the rule of law but of a judicial system acting on instructions."

The arrests in Izmir come amid a months-long legal crackdown on the opposition that had been focused on Istanbul.

A similar operation in Turkey's largest city on March 19 saw the arrest of Mayor Ekrem Imamoglu, the principal rival of President Recep Tayyip Erdogan.

Imamoglu was jailed in March pending trial on corruption charges, which he denies.

Last month he appeared in court on charges of threatening, insulting, and targeting a public official.

Imamoglu's arrest sparked the largest street protests in a decade and a sharp selloff in Turkish assets.

While opposition parties have accused Erdogan's ruling Justice and Development Party (AKP) of making use of a politicized judiciary to crack down on dissent, the government denies the charges, saying that judges are independent. 

Edited by: Saim Dušan Inayatullah