A long-delayed free trade agreement between the United States and South Korea took effect Thursday, winning praise from the two countries' leaders and prompting rallies by supporters and opponents.

Export-dependent South Korea now has trade pacts with the world's two biggest economic areas, after a deal with the European Union went into force in July.

The latest pact, which scraps duties on thousands of items, is the biggest for the United States since the North American Free Trade Agreement in 1994.

US President Barack Obama called his South Korean counterpart Lee Myung-Bak to welcome the beginning of the agreement, which analysts say will boost exports by billions of dollars a year and strengthen the decades-old alliance.

Lee told Obama the deal will be a "good model" for global free trade and strengthen economic development and bilateral ties, according to his office.

The agreement was originally signed in July 2007 but approved by the US Congress only in October, after a partial renegotiation to address US auto industry complaints.

South Korea's parliament ratified it in November despite vehement protests from opposition lawmakers, one of whom set off a tear gas canister in the assembly.

Lee said the world envies South Korea's latest trade deal and urged officials to help farmers, fishermen and small-to-medium-sized firms adapt to it as soon as possible.

"Instead of simply compensating them for their losses, we have to help them gain a competitive edge," Yonhap news agency quoted him as saying.

US Secretary of State Hillary Clinton said the treaty would benefit the United States strategically as it puts a renewed focus on fast-growing Asia.

"Not only will the agreement provide a significant economic boost to both of our economies, it will strengthen the US partnership with a key ally in a strategically important region," Clinton said in a statement.

"This is a powerful signal of America's commitment to the Asia Pacific and to securing and sustaining our role as a regional leader and Pacific power."

Critics say the deal is lop-sided and serves big business at the expense of South Korea's farmers and service industries.

Seoul's foreign ministry said in a statement it would expand trade and strengthen security ties with its closest ally. The US has 28,500 troops stationed in the South to deter a North Korean attack.

Two-way trade was worth $101 billion in 2011, up from the previous year's $90.2 billion, according to Korea Customs Service data. And the US International Trade Commission has estimated that Korean exports to the United States would increase by $6.4-$6.9 billion annually following the deal.

South Korea's main opposition party, which negotiated the 2007 deal while in office, says revisions have made it one-sided. It vows to seek a renegotiation if it wins a general election next month and a presidential poll in December.

An estimated 1,200 protesters rallied in central Seoul Wednesday evening, hours before the agreement came into force at midnight local time.

On Thursday some 30 people including politicians and farmers' and workers' representatives demonstrated against it, while an umbrella anti-FTA group called for a candlelit vigil for the evening.

Protesters said the agreement would only "make the rich richer and the poor poorer" and devastate Korea's weak agriculture and service industries.

But hundreds rallied outside Seoul station in support of the deal. "Let's occupy the world market and become an economic power," read one slogan.

South Korea also has trade deals in force with the EU, India, Chile, Peru, the Association of Southeast Asian Nations, Singapore and the European Free Trade Association (Liechtenstein, Iceland, Norway, and Switzerland).

It has also agreed to start formal negotiations as soon as possible with its largest trading partner, China.