Agreement with collective noun

1) There is a big crowd of people. - correct

2) There is a big crowd of people gathering outside the government house. - correct

3) A big crowd of people are gathering outside the government house. – correct

Comparison:
There was a good crowd on the first night of the show. (from Longman Dictionary)

A big crowd of demonstrators were protesting against cuts in health spending. (from Cambridge dictionary)

A capacity crowd of 76,000 people was at Wembley football stadium for the event. (Collins Dictionary)

The verb should agree with the subject. “A crowd” is the subject, not people. “of people” is a prepositional phrase. Do not make the verb agree with the object of preposition. Collective nouns may be either singular or plural, depending on the meaning of the sentence. You should determine agreement for each collective noun on sentence-by-sentence basis. If the sentence implies that the group name by the collective noun acts as a single unit, use a singular noun. If the sentence implies that the group named by the collective noun acts individually, use a plural verb. In America, a collective noun usually takes a singular verb. If, however, the individuals or parts of the group are considered, the verb is plural.

The jury [a unit] is going to reach a verdict before six o’clock.
The jury [members] were arguing themselves. [Jury is thought as individuals.]

The committee has voted to lay the motion on the table.
[The committee acts as a body and is singular in meaning]

The committee differ, some supporting the notion and some opposing it.
[“differ” indicates the plural sense of the committee.]

Comment:
你上面三句子, a big crowd of people, 有少少 redundant. Crowd 已經解 a large group of people, a crowd of protesters, demonstrators, a crowd of 30,000 比較好些 (more specific), 只是愚見.

2012-07-24 10:00:03 補充：
Typo error:
The committee differ, some supporting the MOTION and some opposing it

2012-07-24 19:06:16 補充：
To 意見者： AhShing”
(2) Both “is” and “are” are acceptable
“are” is acceptable when considering a lot of people as individuals (Br.)

But we seem to say:
There is a ....., There is an ......, There is one....
There are many.., There are two....
除了 There are a lot of (a number of) + plural noun

2012-07-24 19:35:53 補充：
There is a difference between US and British usage.

A crowd is restless (US) = A crowd are restless (Br.)

Well, I just put in my two cents!

2012-07-28 10:23:46 補充：
Correction:

If the sentence implies that the group name by the collective noun acts as a single unit, use a singular VERB. ( ~ not noun)

"crowd" could be quantified by "big" and "small". I don't think it is redundant!

2012-07-24 11:19:12 補充：
Well quoted, Godfry! But, there was a inconsistency between what you quoted and what you answered.

If (3) is correct (i.e. acting individually), then (2) should be incorrect.

(2) should be: There are a big crowd of people gathering ....

A big crowd (are); 30,000.
A capacity crowd (is).