TUNIS (Reuters) - Rail, bus and air traffic stopped in Tunisia and street protests drew thousands on Thursday as the huge UGTT union staged a one-day nationwide strike to challenge the government’s refusal to raise the salaries of 670,000 public servants.

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The strike was the biggest since the assassination of opposition politician Chokri Belaid in February 2013, when Tunisia was navigating a rocky transition to democracy after the toppling of autocrat Ben Ali two years earlier.

The North African country has since adopted a constitution guaranteeing fundamental rights and holding free elections, making it the only “Arab Spring” country to avoid conflict as in Syria or further bouts of political turmoil like Egypt.

An economic crisis has eroded living standards for Tunisians, however, and unemployment is high as political turmoil and lack of reforms have deterred investment needed to create jobs. That has forced the government to launch austerity measures to please donors and lenders including the International Monetary Fund.

“The government have chosen the confrontation with the public servants and we are ready,” UGTT leader Nourredine Taboubi told thousands of people gathered in front of UGTT headquarters on Thursday.

“We will study on Saturday the next steps and we will step up our action and will not back down”, he added without giving details on a possible escalation.

Protesters outside the UGTT HQ and in the capital’s Habib Bourghiba Avenue called for salary hikes.

“The people want the overthrow of the government”, “Power belongs to the people”, they chanted -- slogans used in 2011 when protests against Ben Ali began the “Arab Spring”.

Police showed a strong presence but did not interfere.

Other cities also saw protests, including Sidi Bouzid, the cradle of the 2011 revolution, Gafsa, Jendouba and Sfax.

The one-day strike hit ports, schools, hospitals, state media and government offices. At Tunis Carthage airport, most flights were canceled and check-in counters closed, leaving hundreds of angry passengers stranded.

Tunisia is under pressure from the IMF to freeze public sector wages -- the bill for which doubled to about 16 billion dinars ($5.5 billion) in 2018 from 7.6 billion in 2010 -- as part of measures to reduce its budget deficit.

But the UGTT says the monthly average wage of about $250 is one of the lowest in the world, while the state Institute of Strategic Studies says real purchasing power has fallen by 40 percent since 2014.

Spokesman Iyad Dahmani said the government did not have the funds to pay more for public employees and that any increase would lift annual inflation to 10 percent from 7.4 percent now.

Government and union sources told Reuters the government had proposed spending about $400 million on pay rises versus $850 million the UGTT wants.

That would help cut the public sector wage bill to 12.5 percent of gross domestic product in 2020 from 15.5 percent now -- one of the world’s highest levels according to the IMF, which struck a $2.8 billion loan deal with Tunisia in December 2016.