Mexican archeologists have found what may be the most significant Aztec ruin in decades, with the unearthing of an altar and a monolith in the busy heart of Mexico City, Mayor Alejandro Encinas said on Wednesday.

The 15th-century altar, part of the Aztec empire's main temple, was uncovered last weekend near the city's main Zocalo square along with the 11-foot (3.5-meter) stone slab, most of which is still buried under earth.

"It is a very important discovery, the biggest we have made in 28 years. It will allow us to find out a lot more," Encinas told reporters.