Plural words/singular modifiers

A simple way of looking at it is: Words that come after a Preposition (e.g. "Of") are considered part of a Prepositional Phrase, and thus they're normally skipped as the "determining" Noun.

In your example, "amount" is the real Subject and not "people."

By the way, we normally say: "number" for people and other Countable Nouns. As in: "There was a huge number of people."

But, "There was a huge amount of water in the tank."

To skip to the answer, both are grammatical in your case. Now, the explanation.

First, the word amount is typically used for mass nouns and for money; these both normally get treated as singular, so amount almost always will take is/was (See Ngram; many of the small proportion of 'were's are from the use of the subjunctive).

For count nouns like people we use number. So I'll analyze the same question for "a huge number of".

My rule: figure out which noun is actually the subject of the verb, and make the verb agree with that noun.

A huge number of people were protesting.

The number wasn't protesting, the people were.

A huge number of people attending does not necessarily make a concert profitable.

It's not the people that don't make the concert profitable, but the number.

A huge number of people were at the concert.
A huge number of people was at the concert.

Both the huge number and the people were at the concert, so my rule would let you use either verb. I would have guessed that were is usually used with "a huge number of" in situations like this, because you're usually more focused on the people rather than on the number. However, Google Ngram shows "were a large number of" and "was a large number of" are both used reasonably often. And looking at the actual instances, most fall into the case that the OP was asking about.

When you get rid of the word "large", the Ngram changes dramatically, and you discover that "were" is used most of the time with "a number of".