South Korea's opposition party will end its boycott of the legislature over resumed U.S. beef imports, an official said Tuesday.

The Democratic Party, which had refused to take part in the new National Assembly session that was supposed to start in late May, has agreed to rejoin parliament starting Thursday, said Suh Nam-kwon, an aide to the party's floor leader.

The move came a day after South Korean President Lee Myung-bak fired three ministers amid the beef crisis, which has paralyzed the government and led to weeks of anti-government rallies over concerns the country could be exposed to mad cow disease.

The public backlash forced Seoul last month to negotiate an amendment to the U.S. import deal, allowing in only beef shipments from cattle younger than 30 months, which is believed less susceptible to mad cow disease.

To help reassure the public, restaurants were required starting Tuesday to identify the origin of beef being served or face fines of up to $2,900. Those who falsely identify beef can be fined up to $29,000 or be sentenced to three years in prison, the Agriculture Ministry said in a statement.

Fast food chains, the military and cafeterias in schools and hospitals are also obliged to label their beef, the ministry said. Smaller restaurants have a three-month grace period to meet the requirement.

To enforce the measures, the ministry will mobilize some 5,000 inspectors.

Also Tuesday, South Korean prosecutors said they were investigating some Internet users for allegedly blackmailing companies into pulling ads from newspapers critical of demonstrations against the resumption of U.S. beef imports.

The Internet users allegedly posted messages on Web sites calling for the boycott of products made by companies that have advertised in the country's major conservative dailies, said Ku Bon-jin, an official at the Seoul Central District Prosecutors' Office.

Ku said prosecutors were looking at whether the Web users violated laws on hindering business operations and blackmail.

Daily candlelight protests have dwindled since the amendment, with only several hundred people rallying against U.S. beef imports in Seoul on Sunday and Monday. Police also began blocking a plaza in front of Seoul's City Hall — the main site of the weekslong demonstrations.

On Tuesday evening, about 1,000 people staged a rally at another Seoul location while only about 50 people gathered at the City Hall plaza, according to police. There were no immediate reports of clashes or violence.