TEHRAN—Iranian authorities warned protesters not to stage further antigovernment demonstrations, as a video appeared to show security forces using tear gas to contain protests over the weekend.

The demonstrations broke out after the Iranian military on Saturday said it unintentionally shot down a Ukrainian passenger airliner last week, killing all 176 people on board, after denying for days that it was responsible. The protesters denounced what they called lying and incompetence by the country’s leadership.

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