Maracana stadium construction workers downed tools on Monday and threatened to strike, putting more pressure on World Cup organisers who face a race against the clock to get the arena finished on time.
The workers, who are demanding a wage increase, meal vouchers and private health insurance for their families, staged a one-day stoppage and threatened a full-blown strike from next week.
The famous arena, which staged the final matches in the 1950 World Cup, is being refurbished for this year's Confederations Cup and next year's World Cup at a cost of 900 million reais ($458 million).
It is due to re-open for a friendly between Brazil and England on June 2 and stage its first competitive match, the Confederations Cup tie between Mexico and Italy, on June 16.
Work has already overshot the original deadline of Dec. 31 and has been progressing almost non-stop, with three daily shifts, as the deadline looms.
"It was a warning for the construction companies," said Wagner Antunes Siqueira, a director of the Rio de Janeiro heavy industry workers' union.
"We are going to wait for negotiations on Friday. If there is no (agreement), we will hold an assembly to vote on an indefinite strike."
Four of the six stadiums to be used for the Confederations Cup, a dress rehearsal for the World Cup, are still not finished. Only Fortaleza's Castelao and Belo Horizonte's Mineirao have so far re-opened.
FIFA secretary general Jerome Valcke warned three weeks ago that there would be no further extension beyond the new deadline of April 15.
"We cannot go beyond this date. There cannot be any further delays. All the stadiums must be ready by then," he said.