Tafsir of Al-Anbiya' 21:61

They said: "Then bring him before the eyes of the people," that is, bring him into their presence, so that he may be observed and witnessed by them in the most complete manner, as is implied by the preposition ‘ala (upon), which is used metaphorically to establish the state of seeing.

"So that they may bear witness," meaning: so that they may witness our punishment of him. It has also been said: that they may bear witness to his deed or his utterance. In that case, the pronoun does not refer to the [general] people, but rather to a specific group among them who are either unspecified or known. The first interpretation is narrated from Ibn Abbas and al-Dahhak, while the second is from al-Hasan and Qatadah; the latter is more consistent with the expectation of [the people's] testimony.