Thousands of people who oppose same-sex marriage rallied on Parliament Hill Saturday, cheering as Conservative Leader Stephen Harper pledged to support the traditional definition of the union.

"I am committed, when I am elected prime minister – at the next election, whenever that may be – to bring in legislation that will define marriage as the union of one man and one woman," Harper said.

About 15,000 demonstrators, including many people who travelled from Toronto and Montreal, gathered to protest against Bill C-38, federal legislation that would extend civil marriage to same-sex couples.

Thousands of people march against same-sex marriage legislation at Parliament Hill on Saturday. (CP Photo)

The Liberal government of Prime Minister Paul Martin drafted the bill after courts in many provinces ruled that keeping gay couples from marrying violated their equality rights under the Charter.

But the proposed law hasn't gone over well with a number of Canadians – including most of the people who gathered Saturday in a demonstration that was part protest, part parade and part prayer service.

Some carried signs with the image of the late Roman Catholic leader Pope John Paul II, while others carried signs with slogans such as "God defined marriage, the government defies God."

Conservative Leader Stephen Harper spoke at the rally, promising to enshrine the traditional definition of marriage if made prime minister. (CP Photo)

"I'm here because I agree with the normal definition of marriage," said protester Oliver Locke.

Fay Chow, who came with hundreds of Chinese-Canadians from Toronto, agreed. "We want all the members of Parliament to know how the majority of Canadians feel about marriage."

Harper told the crowd that all but four Conservative MPs support the traditional definition of marriage.

"Marriage belongs to us, not Paul Martin, and we will defend it," said Conservative MP Jason Kenney.

A small but vocal group at the rally supported same-sex unions.

"This is an issue for all Canadians about equality according to the Charter," said Prudence Craib.

Harper has introduced an amendment to Bill C-38 that would reverse the intent of the act by specifying that marriage must continue to be defined as a union between one man and one woman to the exclusion of all others.

Parliament is to vote on the amendment on Tuesday.

A number of rallies, both in support of and in opposition to same-sex marriage, are planned in cities across the country before that vote.