* Unions draw a fraction of the crowds at June protests
* Demonstrations briefly block major roads in many cities (Adds details on clashes with police late in the day)
By Lucas Iberico-Lozada and Brad Haynes
SAO PAULO, July 11 (Reuters) - Tens of thousands of union demonstrators blocked roads and snarled traffic in dozens of Brazilian cities on Thursday in a one-day strike aimed at seizing the momentum of huge protests that swept the country last month.
The disruptions, mostly peaceful aside from scattered clashes between police and protesters in a few cities, were limited compared to June protests that drew more than 1 million supporters in Brazil's largest demonstrations in two decades.
Late Thursday, police in Rio de Janeiro fired tear gas and used batons to disperse an unruly group of demonstrators, some of them hooded and masked, that sought to take advantage of the march by union workers to create unrest. Labor activists also tried to push back some of the aggressive demonstrators.
Bus drivers, metal-workers, stevedores and bank tellers were among the unionized workers who took to the streets nationwide as organized labor attempts to elevate workers' rights above the tide of complaints that washed across Brazil in recent weeks.
The earlier wave of discontent has quieted in recent weeks after sending shockwaves through the political landscape and spurring President Dilma Rousseff to pledge reforms and more investment in public services.
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 was sidelined in the June demonstrations, but is now trying to assert leadership and provide direction for what was a diverse and leaderless protest movement.
Protesters on Thursday demanded more labor rights while echoing the calls last month for greater investment in public health, education and transportation. Brazil's biggest unions represent about one-tenth of the workforce.
Leaders of Brazil's largest umbrella union, known as CUT, which provides support for Rousseff's leftist Workers' Party, aimed to channel popular discontent and counter the notion 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.
The biggest gatherings paled in comparison to the protests last month, which packed major boulevards.
Sao Paulo's highest-profile union rally drew a few thousand paticipants at its peak along four blocks of Avenida Paulista, the city's main avenue. Police cordoned off the area and diverted traffic to allow the march to proceed.
Office worker Hermes Pereira watched skeptically from the margins of the rally. "The protests last month made demands without a political platform, without a party. Today they're demanding the same things as last month, but without the same strength," he said.
The previous protests subsided into smaller, isolated demonstrations after swift government concessions, isolated violence and a lack of central leadership, raising questions about their lasting impact.
Demonstrators on Thursday briefly blocked dozens of major roads around the country, setting tires ablaze on a freeway outside Rio de Janeiro and marching over a suspension bridge in Sao Paulo's financial district.
Dock workers joined the strike, holding up a sugar vessel and several container ships at Santos south of Sao Paulo, the busiest container port in South America.
But there were notable absences - subway and train workers in Sao Paulo and Rio did not take part, minimizing the impact in Brazil's two biggest cities.
It was a largely normal workday in the national capital Brasilia, despite a demonstration by several thousand that diverted traffic. A few hundred landless peasants also occupied land reform institute INCRA to demand agrarian reforms.
In parts of the country with a strong history of union activity, such as Porto Alegre in the far south, public transportation, stores and even intercity buses shut down.
Rousseff, who is widely expected to run for re-election next year, is trying to balance the demands for reforms with her government's renewed vows of fiscal responsibility. The protests badly damaged her approval ratings.
Looser fiscal policy, rising inflation and weak economic growth have stoked investor concerns about Brazil's economy, triggering a broad selloff on the Sao Paulo stock exchange and warnings of a lower sovereign debt rating. (Additional reporting by Caroline Stauffer and Eduardo Simoes in Sao Paulo, Maria Pia Palermo and Rodrigo Viga Gaier in Rio de Janeiro, Raquel Stenzel in Porto Alegre; Editing by Todd Benson and Paul Simao)
Our Standards: The Thomson Reuters Trust Principles.