Up to 600 people may have been killed in last week's clashes in the Democratic Republic of Congo's capital, Kinshasa, European Union diplomats say.
Soldiers and civilians brought bodies to city mortuaries
The authorities have said about 60 people died in violence between the army and armed guards loyal to opposition leader Jean-Pierre Bemba.
The EU diplomats expressed concern at the government's "premature" use of force which they say led to fighting.
President Joseph Kabila said his troops put down an armed rebellion.
The violence threatened to derail the peace process which ended DR Congo's war and led to elections last year.
Mr Kabila defeated Mr Bemba, a former rebel leader, in a second round run-off.
But Mr Bemba refused to have his armed bodyguards integrated into the national army before last week's deadline.
The German ambassador to Kinshasa told reporters that figures collated from hospitals and morgues indicated that up to 600 people could have died in the violence.
"There were enormous numbers of civilian casualties. Shells fell on the district, on homes. We will probably never know the exact number of victims," Britain's ambassador Andy Sparkes said.
Aid group Caritas had earlier put the number at 150.
Earlier, the United States condemned the violence.
"[It] represents a set-back in the progress the Congolese people expect and deserve after last year's historic elections," US State Department spokesman Tom Casey said in a statement.
Mr Bemba's allies have condemned what they described as the arbitrary arrest and intimidation of its members.
In a statement the EU ambassadors called on Mr Kabila's government "to do everything to assure the existence of a democratic space in order to guarantee the free expression of all political opinions", AFP news agency reports.
But Mr Kabila defended the army's role on Monday and he dismissed calls to negotiate with Mr Bemba, who has taken refuge in the South African embassy.
As a senator, Mr Bemba, who has been charged with treason, enjoys immunity from prosecution. The government says it will seek to have this stripped.
Mr Bemba denied plotting military action to overthrow the president and accused the army of trying to kill him.
As a former vice-president in the transitional government, Mr Bemba is entitled to 15 policemen for his protection.
Under another agreement signed ahead of the election, the winner of the presidential poll is committed to guarantee the loser's security.