TUNIS - 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.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.