BEIRUT, Lebanon — Spillover from the Syrian conflict hit Lebanon in a frightening new way on Wednesday with the abduction of more than 30 Syrians inside Lebanese territory, which their captors called revenge for the kidnapping of a relative inside Syria.

Members of a powerful Lebanese Shiite family who captured the Syrians, displayed in a video shown on Lebanese television, threatened to cause havoc in the streets and conduct an extended kidnapping spree in Lebanon until their family member, taken hostage by Syrian rebels, was set free.

“The next few hours and next few days will determine what will happen,” one masked captor said outside a family property in a Shiite suburb of Beirut. Another member of the family that captured the Syrians, Hatem al-Mikdad, brother of the Lebanese hostage in Syria, said that if he was not released soon, “we will kill all of them.”