Image caption Mexico's left-wing Democratic Revolution Party has opposed the measure

Mexico's Senate has approved a measure to open the state-run oil fields to foreign investment for the first time in 75 years.

The measure would let private firms explore and extract oil and gas with state-run firm Pemex, and take a share of the profits.

It now moves to the lower house to be voted on, where it is expected to pass.

President Enrique Pena Nieto wrote on Twitter that it was "a significant decision for Mexico".

Mr Pena Nieto said it was necessary to modernise Mexico's energy sector and increase oil production, which has dropped from 3.4 million barrels per day in 2004 to the current rate of 2.5 million barrels per day.

However, the left-wing Democratic Revolution Party said it was a submission to US oil companies, and protesters set up camp outside the Senate.

They say the move strikes at the heart of Mexico's identity.

In 1938, then-president Lazaro Cardenas nationalised the oil industry, which had been operated by foreigners up to that point, asserting that Mexico had a right to its mineral wealth.

If the measure passes Mexico's Congress, it must then be approved by 17 of the country's 32 federal entities.