According to Twitter reports, the explosion came from a trash can in Toontown; an evacuation is underway.

Something went BOOM in Toontown at Disneyland and now they're evacuating. #Disneyland pic.twitter.com/in5FadUu3i — Allen Wolf (@theallenwolf) May 29, 2013

Fire crews are reportedly on the scene:

#Breaking: Explosion in Trash Can Reported at Toontown in Disneyland. Fire Crews on Scene. — KTLA (@KTLA) May 29, 2013

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Update, 9:10 pm ET:

It’s still early, but at this point we do not see any reports of injuries.

#Disneyland evacuation seems orderly, no panic nor injuries reported. "Explosion" may be too strong of a word to describe event. — Bob Owens (@bob_owens) May 29, 2013

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Update, 9:12 pm ET:

Nothing about the incident on Disneyland’s Twitter feed as yet.

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Update, 9:18 pm ET:

Via L.A. Times reporter Andrew Blankstein, the explosion may have been caused by dry ice in a plastic bottle:

Breaking: Explosion at Disneyland. Police tell us it appeared to involve dry ice. Post soon @LANow http://t.co/OluaZmcbxs — robert lopez (@LAJourno) May 29, 2013

Wikipedia has an entry on a “dry ice bomb” here.

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Update, 10:12 pm ET:

Disney has released an official statement via Twitter confirming that there were no injuries and that the area was evacuated “in an abundance of caution.”

This afternoon, a small bang was heard in a trash can at Mickey's Toontown. In an abundance of caution, the area was evacuated. (1/2) — Disneyland Resort (@Disneyland) May 29, 2013

There were no injuries and no reported damage. We are working with local authorities who are now on-site and reviewing. (2/2) — Disneyland Resort (@Disneyland) May 29, 2013

After an investigation by authorities, the park has given the all-clear.