UPDATE 2-Rousseff rises in Brazil poll; Serra hits at crime

* Rousseff lead in poll expands to 17 points over Serra

* Valid vote count would give Rousseff 1st round win

* Serra refuses poll comment, criticizes violence (Adds Rio violence, Serra comments)

By Elzio Barreto

SAO PAULO, Aug 21 (Reuters) - Ruling party presidential candidate Dilma Rousseff extended her lead over rival Jose Serra after taking advantage of free broadcast advertising, a poll showed on Saturday.

Rousseff, President Luiz Inacio Lula da Silva's former chief of staff, jumped to 47 percent of voters' support from 41 percent last week -- her biggest lead yet in a Datafolha poll.

Serra, former Sao Paulo state governor and health minister, fell 3 percentage points to 30 percent of voter intention.

The 6-point surge for Rousseff in advance of October's election was mostly credited to increased exposure after the presidential candidates took to the airwaves on Tuesday with free television and radio advertising, Datafolha said.

"Serra's campaign seems to have received its death certificate with the release of the Datafolha poll, which shows an enormous difference in favor of Dilma Rousseff (from the Workers' Party)," Folha de S.Paulo newspaper, Brazil's largest daily, said in an editorial on Saturday.

With recent poll results increasingly showing Rousseff winning in the first round, she might have a stronger mandate to push through her legislative agenda.

"It gives greater legitimacy for Dilma," said Cesar Alexandre Carvalho, a partner at consultancy CAC Consultoria Politica in Brasilia. "A woman president, coming to power in a first-round vote, obviously she will have greater bargaining power. Now, what remains to be seen is what she will do with that, how it will be used to approve certain projects."

Not even Lula, a former union leader known for his charisma and off-the-cuff remarks, won in a first-round vote in the 2002 and 2006 elections, suggesting Rousseff could have an easier time putting together a coalition in Congress.

"TV proves once again its power to reach and penetrate into the most diverse levels of the Brazilian population, including those where access to information is scarce," Datafolha directors Mauro Paulino and Alessandro Janoni said in an article in Folha de S.Paulo newspaper detailing the poll results.

The figures followed other poll results during the week showing Rousseff with more than the 50 percent needed for a first-round victory if abstentions and blank votes are excluded, as they are in the election.

Former environment minister Marina Silva garnered 9 percent of votes in the poll, down from 10 percent in the last survey.

Datafolha polled 2,727 people nationwide on Aug. 20. The poll has a margin of error of plus or minus 2 percentage points.

Serra refused to comment on the poll results during a campaign visit to a hospital in Rio de Janeiro state. Instead, he criticized the federal and state governments for not doing enough to clamp down on drug-related crime in Rio and called a shootout between police and suspected members of a gang outrageous.

With Brazil's economy expanding at a fast pace and unemployment near all-time lows, violence could increasingly become a hot topic in the political campaign.

"An event like this harms Rio's image abroad," Serra said. "A governor that isn't doing his job, wakes up crime in other states," he added in reference to Rio Governor Sergio Cabral, who is a Lula ally and whose party is part of the ruling coalition.

He added that Brazil doesn't have an effective program to fight drug and arms trafficking and reiterated calls for the creation of a Public Safety Ministry to better handle those issues.

Police in Rio de Janeiro arrested 10 heavily armed men and rescued 35 people who were held hostage for almost two hours at a five-star hotel. (Additional reporting by Rodrigo Viga Gaier in Rio de Janeiro; Editing by Bill Trott and Jackie Frank)