Tens of thousands of people have protested in the centre of Mexico City against President Enrique Pena Nieto's planned overhaul of the energy sector.
Opposition leader Andres Lopez Obrador told the crowd to surround the Congress this week.
Mr Pena Nieto says the plan to allow private investment in the oil and gas sector is needed to boost the economy.
His approval ratings have slumped to their lowest since he took office a year ago.
A recent opinion poll suggested Mr Pena Nieto's approval rating had fallen to 44%, eight percentage points lower than in July, with 48% of those polled disapproving of his administration.
'As long as it takes'
"We will carry out the sieges we have agreed upon in the Senate, the Lower House and the local assemblies. I will be there for as long as it takes," Mr Lopez Obrador told thousands of protesters in Zocalo square.
Sunday's protest gathered supporters of the defeated presidential candidate Mr Lopez Obrador as well as teachers and union members.
If passed, the government's energy bill will encourage foreign and domestic investment in the industry.
A source of national pride, Mexico's oil industry has been protected from private involvement for 75 years.
The overhaul is part of wide-ranging structural reforms proposed by the Pact for Mexico alliance, led by President Pena Nieto.
The broad alliance has already passed telecommunications reforms and a tax reform bill imposing new levies on junk food and soft drinks.
Mr Pena Nieto also aims to boost government revenue by increasing the top rate of income tax.
He has also passed education reforms which deeply angered some teachers' unions.