BANGKOK, June 7 (Reuters) - Thailand kicked off a long-awaited mass vaccination campaign on Monday (Jun 7) as the country battles its third and worst wave of the COVID-19 epidemic.

The government aims to administer 6 million doses of locally-made AstraZeneca and imported Sinovac vaccines this month, hoping to assuage worries about the slow roll-out and supply shortages.

"The government will ensure that everyone is vaccinated," Prime Minister Prayuth Chan-ocha said in televised comments after he visited an inoculation centre in Bangkok.

The government plans to vaccinate 70 per cent of Thailand's population of more than 66 million people by the end of the year. So far, 2.8 million people deemed most vulnerable, including frontline health and transport workers, have received a first dose.

But the government has come under fire from opposition politicians who accuse it of complacency and an over-reliance on the locally-made AstraZeneca doses.

Thailand had escaped the worst of the pandemic as it hit other countries hard last year, but is now grappling with its deadliest outbreak. The third wave has accounted for more than 80 per cent of total infections so far. Officials reported 2,419 new COVID-19 cases and 33 deaths on Monday, bringing the total to 179,886 and 1,269 fatalities.

"My feeling is that no matter what, we will need to go outside of home for the littlest things, so getting the vaccine gives us a sense of relief," said Praepawee Lertpongkijja, 38, at a Bangkok vaccination centre.

The government has been scrambling to source more vaccines as concerns emerged about the production capacity of royal-owned Siam Bioscience, the Thai company making the AstraZeneca vaccine locally, after the Philippines said its order had been reduced and delayed.

Thailand will receive an additional 3.24 million doses from AstraZeneca after mid-June, the health ministry said in a statement, but did not specify how much would be locally made.

The Thai government received 1.8 million locally-produced AstraZeneca shots on Friday and has sourced another 200,000 doses from South Korea, a health ministry source told Reuters. Thailand expects to sign a contract this week for 20 million shots of the Pfizer/BioNTech vaccine.