Leftist lawmakers locked themselves inside Mexico's chamber of deputies Wednesday to prevent the lower house from debating a controversial oil reform bill that passed the Senate hours earlier.

Around 20 lawmakers took over the chamber's podium, unfurling a banner with the words "traitors" in big letters after wrapping chains around the handles of a door and piling up chairs.

"We cannot allow a handful of people to take and give away the nation's property," deputy Maria Luisa Alcalde of the Citizen Movement Party said from the podium, joined by members of the Democratic Revolution Party (PRD) and the Workers Party.

The bill moved to the lower chamber after the Senate voted in favor of the reform championed by President Enrique Pena Nieto.

The legislation aims to open the state-controlled energy sector to private investment, ending the 75-year-old oil monopoly.

The legislation would let private firms explore and extract oil and gas, as well as share profits, production and risk with the state-run energy giant Pemex, ending a ban cemented in the country's very constitution.

The proposed constitutional changes stem from a deal between Pena Nieto's centrist Institutional Revolutionary Party (PRI) and the conservative opposition National Action Party (PAN).

Analysts say the bill goes further than Pena Nieto's original proposal. Supporters of the reform say it is badly needed to revive Pemex, give Mexico the tools to drill for more gas and oil and reverse a trend of falling production.

But leftist opponents have branded the plan treason and a submission to US oil companies, calling the legislation a bid to privatize a symbol of national sovereignty.

Watch this report from CNN Mexico, aired on Wednesday, below.