* Protests escalate before June 5 presidential runoff
* More than 5,000 farmers protest near Bolivia border
LIMA May 27 (Reuters) - Hundreds of demonstrators mobbed government buildings and burned police cars in southeastern Peru on Thursday as a protest against mining firms intensified 10 days before a presidential election.
Some 5,000 protesters have descended on the city of Puno over the past two weeks to demand concessions be revoked for mining companies they say will contaminate their lands. Roads to neighboring Bolivia are now blocked, paralyzing commerce.
"They've started to loot public and private institutions, banks and shopping centers," police officer William Anda said on local radio.
President Alan Garcia earlier this week authorized the army to help maintain order in Puno, 620 miles (1,000 km) south of Lima, but it has yet to use force to end the protests. The government has sent representatives to negotiate with the protesters but an agreement has not been reached.
Garcia's government has helped line up $40 billion in investments in mining and oil projects over the next decade.
Intent on averting a violent clash that could overshadow the election, Garcia has said the government would not try to stop the protests until after the June 5 presidential vote. Polls give right-wing lawmaker Keiko Fujimori a narrow lead over leftist Ollanta Humala in the runoff.
Both candidates pledge to solve social conflicts over natural resources in Peru. Analysts say protests are caused partly because communities do not feel they have benefited from Peru's mineral wealth and decade-long economic boom. The conflicts frequently turn violent.
Several small precious metals miners operate near Puno as well as Minsur (MINi.LM), Peru's largest tin miner. (Reporting by Marco Aquino, Writing by Caroline Stauffer; Editing by Peter Cooney)