Defence department spokesman Peter Galvez released to the media an aerial photograph of what he said were about 30 blocks on Scarborough Shoal in the South China Sea.
"It's unfortunate that they keep on doing activities that do not contribute to our pursuit towards regional peace," Galvez told reporters.
Defence Secretary Voltaire Gazmin briefed members of parliament about the issue today, telling them the concrete blocks were a "prelude to construction", according to Galvez.
AFP could not immediately verify the photograph. When asked for comment, Chinese embassy spokesman Hua Zhang told AFP by email, "I will look into it."
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Scarborough Shoal is about 220 kilometres off the main Philippine island of Luzon, within the country's internationally recognised exclusive economic zone.
The outcrop is about 650 kilometres from Hainan island, the nearest major Chinese land mass.
China claims most of the South China Sea, including waters close to the coasts of the Philippines and other neighbours.
The Philippines says China has effectively occupied Scarborough Shoal, home to rich fishing grounds, since last year by stationing vessels there and banning Filipino fishermen.
Malaysia, Brunei and Taiwan also have competing claims to parts of the South China Sea, and the rivalries have been a source of tension for decades.
Diplomatic relations between the Philippines and China, in particular, have become increasingly tense in recent years.
China rejects international arbitration, preferring to deal with the issue on a bilateral basis while maintaining it has sole territorial rights.
Legislator Walden Bello, who attended Gazmin's briefing on today, told AFP Filipino politicians were concerned China could be laying the foundations for a military garrison on Scarborough Shoal.