South Korea has held funerals for the two marines killed when North Korea fired artillery shells at the South's Yeonpyeong island earlier this week.
In a service broadcast nationwide, the marines' commander vowed "thousand-fold revenge" for the attack.
Two civilians also died in the incident - one of the worst clashes since the end of the Korean War.
Tension is high, with the North calling planned US-Southern military drills on Sunday an "unpardonable provocation".
The US has called on China to increase its pressure on Pyongyang to prevent further incidents.
China has said its "top priority" is to keep the situation under control. Beijing has begun a series of talks with Washington, Seoul and Pyongyang in an attempt to ease the tensions.
Flag burned
The funeral for marines Seo Jeong-woo and Moon Kwang-wook was held at a military hospital in Seongnam, close to the capital, Seoul.
Hundreds of government and military officials, politicians, religious leaders, activists and civilians attended.
Among them were Prime Minister Kim Hwang-sik and Maj Gen You Nak-jun, the head of the marine corps.
"We'll certainly repay North Korea a thousand-fold for killing and harming our marines," said Gen You.
"South Korean active-duty marines and all reserve forces will engrave this anger and hostility in our bones and we will make sure we take revenge on North Korea."
Officials and relatives placed white flowers on the two coffins draped in the South Korean flag. Marines sang as the coffins were carried out.
Elsewhere in Seoul, 1,000 South Korean military veterans protested, burning the North Korean flag and portraits of Pyongyang's leaders. The protesters shouted slogans demanding revenge and condemning the North's "atrocity".
'Uncontrollable'
The four South Koreans died on Tuesday when the North launched a sudden barrage of shells at Yeonpyeong island, close to the maritime border between the two countries.
The attack led to the South replacing its defence minister and evacuating most of the island's civilian population.
The new Defence Minister Kim Kwan-jin said on Saturday that tougher action was needed against the North.
"We need to deal with North Korea's provocations strongly. We need to hit back multiple times as hard."
Pyongyang said it had been provoked by the South's military exercises, which were being carried out close to Yeonpyeong.
China, Pyongyang's only ally, has been increasing diplomatic efforts to ease tensions on the peninsula and avoid further violence.
On Friday, China's Foreign Minister Yang Jiechi met the North's ambassador in person, and spoke on the phone to his US and South Korean counterparts, according to the state-run Xinhua news agency.
"The top priority now is to keep the situation under control and to ensure such events do not happen again," a statement said.
The top US military commander, Adm Mike Mullen, said he did not know "why China doesn't push harder" with Pyongyang.
In an interview with CNN due to be broadcast on Sunday but released as a transcript, Adm Mullen said Beijing appeared to mistakenly believe it could control North Korea's leader, Kim Jong-il.
"I'm not sure he is controllable," Adm Mullen said.
The US and South Korea are due to begin four days of pre-arranged military exercises on Sunday.
China said they would escalate tension and has warned against any infractions into its exclusive economic zone, which extends 320km (200 miles) from its coast.
Pyongyang said the drills were pushing the region to "the brink of war". On Saturday state media promised a "sea of fire" if North Korean territory was violated.
The Pentagon has said Beijing is being kept informed about the naval exercises.
South Korea has increased its troop numbers on Yeonpyeong and says it will change its rules of engagement to allow it to respond more forcefully if incidents such as Tuesday's happen again.
This week's tension comes as the North is undergoing an apparent transition of power from Kim Jong-il to his young son Kim Jong-un.
