The Chief Minister was given a warm welcome at the onset of the function.

Onset : the beginning of something, especially something unpleasant.
Hindrance : a thing that provides resistance, delay, or obstruction to something or someone.
Commencement : the beginning of something.
Finale : the last part of a piece of music, an entertainment, or a public event, especially when particularly dramatic or exciting.
Conclusion : the end or finish of an event, process, or text.

Antonym of Onset is Finale