Subway workers in Sao Paulo went on strike Thursday, one week before the World Cup opens in the Brazilian mega-city, where protests against government spending on the tournament erupted a year ago.

The Sao Paulo metro is the main transport link to the economic capital's World Cup host stadium, and the indefinite strike could pose a massive logistical headache for organizers.

The strike will affect 4.5 million daily passengers and could unleash commuter chaos in the sprawling city of 20 million people, which was already hit by a paralyzing bus drivers' strike last month.

Workers went on strike at midnight after negotiations on a salary increase fell through. They rejected an offer of 8.7 percent, insisting on at least 10 percent, said the president of their union, Altino Melo dos Prazeres.

"If there's money for the Itaquerao (the nickname of host stadium Corinthians Arena) and the World Cup, how is it they don't have any money for public transport?" said Prazeres, quoted by newspaper O Estado de Sao Paulo.

He said any offer would have to be in the double digits to keep up with inflation -- a growing concern in Brazil, which is struggling with slowing growth and rising prices.

Brazil has been gripped by a wave of strikes and protests ahead of the World Cup and elections in October.

The movement erupted one year ago in Sao Paulo with protests against a transport fare increase that soon ballooned into a mass show of anger over the more than $11 billion being spent on the World Cup in a country with pressing needs in education, health care, transport and housing.

A million protesters flooded the streets last June during the Confederations Cup, a World Cup dress rehearsal.

The protests turned violent at times, overshadowing the tournament and raising fears of a repeat this year.

- New protests -

Many say the national mood is largely the same.

"Nothing has changed. The people took to the streets and no level of government has managed to respond to their demands," said Antonio Carlos Costa, founder of activist group Rio da Paz (River of Peace).

"Brazil has the seventh-largest economy in the world, but it ranks 85th on the human development index, there are 50,000 murders a year. It's normal for people to get angry when so much money is being spent building stadiums."

The protests have continued since last June, but on a much smaller scale, organized by veteran protest groups.

"It's radically different from what happened in 2013, when the protests were an expression of the country's malaise and people took to the streets spontaneously. Now the malaise is still there, but parties and movements have taken the lead and the middle class is staying away because they're afraid of the radicals' violence," said Jose Augusto Rodrigues, a sociologist at Rio de Janeiro State University (UERJ).

A case in point, about 12,000 protesters rallied by the Homeless Workers' Movement (MTST), a mainstay of the anti-World Cup protests, marched on Corinthians Arena Wednesday, joining around 400 retired military police and their relatives who were calling for higher pensions.

President Dilma Rousseff has defended her government's preparations for the tournament, insisting that the money spent will leave a legacy of airports and transportation infrastructure that will benefit Brazil long after the tournament.

But the government has also faced criticism for chronic delays and disorganization.

Workers are still scrambling to finish several of the 12 host stadiums, including Corinthians Arena, which has not had all its seats installed.

Much of the other promised infrastructure has been shelved, from roadworks to a high-speed train to subway and monorail lines.

Meanwhile, with just over a week to go, teams continue trickling into Brazil. Chile are due to arrive later Thursday, joining Australia, Croatia and Iran.

Brazil, who routed Panama 4-0 in a friendly Tuesday, play Serbia on Friday in a final warm-up game before taking on Croatia in the kick-off match.