Amid cries of defiance and a cloud of tear gas, South Korea has ratified a controversial trade treaty with the U.S.

The passage of the Korea-U.S. free trade agreement effectively opens the Asian country’s legal market to foreign law firms, most of which had effectively been barred from practicing there.

Following Tuesday’s ratification, at a parliamentary session marred by protest including a tear gas canister set off by an opposition politician, U.S. firms will be able to set up Korea offices from Jan. 1, 2012. The U.S. Congress approved the treaty last month.

Among the firms expected to enter the country are Paul Hastings, which has already stated its intention to do so. Cleary, Gottlieb, Steen & Hamilton and Simpson Thacher & Bartlett run leading Korea practices from their Hong Kong offices, although neither firm has yet announced plans for an office in Korea.

The treaty also clears the way for U.K. firms to open offices in Seoul. Although British firms have been allowed to enter Korea under an FTA between South Korea and the European Union that has been in force since July, none has done so. The rules governing foreign legal consultants in Korea require individual lawyers to have at least three years’ experience practicing the law of the country where their firm is based. Most lawyers in U.K. firms’ Korea practices are U.S.-qualified and don’t meet this requirement.

Foreign firms that wish to set up offices in Korea must first gain approval from the Ministry of Justice before registering with the Korean Bar Association; the process should take around three months. Each FTA provides for a five-year period during which firms from the respective countries will not be able to form affiliations or merge with Korean firms, or hire Korean-qualified lawyers.

However, some lawyers have expressed doubt that some key figures will move to Seoul in the near future, citing family commitments or difficulties with cultural integration.