TULTEPEC, Mexico — A powerful chain-reaction explosion ripped through Mexico’s best-known fireworks market on the northern outskirts of the capital Tuesday, killing at least 29 people, injuring scores more and sending a huge plume of charcoal-gray smoke billowing into the sky. The blast leveled the open-air San Pablito Market in Tultepec in the middle of the afternoon as it bustled with shoppers stocking up on fireworks to celebrate Christmas and New Year’s, reducing vendors’ stands to piles of rubble, ash, and charred metal. It was the third devastating explosion and fire to ravage the market since 2005.Crescencia Francisco Garcia said she was in the middle of the grid of stalls along with a few hundred others when the thunderous explosions began. She froze, reflexively looked up at the sky and then took off running through the smoke once she realized everyone was doing so. As she ran she saw people with burns and cuts, and lots of blood. “Everything was catching fire. Everything was exploding,” Francisco said. “The stones were flying, pieces of brick, everything was flying.”In comments broadcast on local TV news, Mexico State Gov. Eruviel Avila reported Tuesday night that in addition to the 26 people who perished at the market, three more victims died after being taken to hospitals. State Health Secretary Cesar Nomar Gomez Monge said 72 people were being treated for injuries including severe burns, in some cases over 90 percent of their bodies. Those hospitalized included 10 children.