* Demonstrations briefly block major roads in several cities

* Union marches less disruptive than protests in June

By Lucas Iberico-Lozada and Brad Haynes

SAO PAULO, July 11 (Reuters) - Union demonstrations snarled traffic in several Brazilian cities on Thursday morning as part of a one-day strike aimed at seizing the momentum of sweeping protests that shook Latin America's largest economy last month.

Bus drivers, metalworkers, stevedores and bank tellers were among the unionized workers who took to the streets in at least dozen of Brazil's biggest cities as organized labor sought to push workers' rights to the forefront of the national debate.

The disruptions, which began during the early morning commute, were limited compared to the mass protests that swept Brazil in June, culminating in its largest demonstrations in two decades and spurring President Dilma Rousseff to pledge reforms.

Union groups played a marginal role in those protests, which were sparked by outrage over higher bus fares and led to demands for better public services, an end to political corruption and more investment in healthcare and education.

Organized labor is now trying to assert leadership and provide direction for what has been a largely leaderless mass protest movement. Unions are asking for shorter work weeks and more generous pensions among other demands.

Leaders of Brazil's biggest umbrella union, known as CUT, which provides key support for Rousseff's leftist Workers' Party, aimed to channel popular discontent while allaying skepticism that they were late to the movement.

"We're not late, we've always been here. We're here to demand that Dilma follows through on her promises," said Adriana Magalh?es, a bank employee and CUT organizer in Sao Paulo.

Demonstrators briefly blocked dozens of major roads around the country, setting tires ablaze on a freeway outside Rio de Janeiro and marching over an iconic suspension bridge in Sao Paulo's financial district.

Dock workers at Brazil's main shipping ports also joined the 24-hour strike, holding up a sugar vessel and several container ships at the port of Santos south of Sao Paulo, the busiest container port in South America.

But there were notable absences - Sao Paulo's subway and train workers voted to abstain from Thursday's walkout.

Union demonstrators demanded expanded labor rights for Brazilians while echoing the calls last month for greater investment in public health, education and transportation.

The growing list of grievances puts Rousseff, who is widely expected to run for re-election next year, in a tough spot as she tries to balance the demands for reforms with her government's renewed vows of fiscal responsibility.

Looser fiscal policy, rising inflation and weak economic growth have stoked investor concerns about Brazil's economy in recent months, triggering a broad selloff on the Sao Paulo stock exchange and warnings of a lower sovereign debt rating.

At the same time, the popular unrest nearly halved Rousseff's approval ratings in a matter of weeks to about 30 percent, casting a sudden shadow over the center-left president's political future.

Rousseff, who took over for the popular former President Luiz Inacio Lula da Silva, has faced criticism from mayors and defiant allies in Congress this week, even as she announced billions in new spending on public transport and other municipal services aimed at quelling popular discontent. (Additional reporting by Caroline Stauffer in Sao Paulo, Maria Pia Palermo and Rodrigo Viga Gaier in Rio de Janeiro; Editing by Paul Simao)

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