The BBC's Roland Buerk in the Sri Lankan capital, Colombo, looks at why journalists and human rights activists there are so angry.
Sri Lanka is one of the world's most dangerous countries for reporters
The police dragged barricades across the road, blocking the route to President Mahinda Rajapaksa's office.
Wearing black armbands, journalists and human rights activists said the freedom of the press had been undermined as the war continues between the government and the separatist Tamil Tigers (Liberation Tigers of Tamil Eelam).
"Media workers are continually being harassed, they are being tortured, they are being hammered," said one man who was holding a placard. "So we have been complaining to the authorities but nothing is happening."
"Perhaps, there are individuals within the heart of the government who feel that any kind of dissent is to be taken personally and squashed, because the end justifies the means," said Paikiasothy Saravanamuttu, from the think-tank the Centre for Policy Alternatives, who had joined the protestors.
"And the end for them may well be their notion of defeating terrorism, but they are destroying everything this country stands for if it is to be a functioning democracy," Mr Saravanamuttu said.
The World Association of Newspapers ranked Sri Lanka the third most dangerous country in the world for media workers in 2007 based on the number who were killed.
Unarmed police officers watched the demonstration from behind the barricades but took no action.
Namal Perera says attackers tried to abduct him
After marching around one of the capital's main roundabouts, severely disrupting traffic, the protesters dispersed.
There have been a series of abductions and assaults against journalists in Sri Lanka, the latest on Monday.
Namal Perera, a defence correspondent and a campaigner, was in a car with a Sri Lankan official from the British High Commission, when they were ambushed.
"I saw the people coming with clubs," Mr Perera said from his bed in hospital.
"They first attacked our windscreen, then they tried to grab me for my shirt. The other persons tried to poke me with the stick. That's how I got these injuries.
"They said in Sinhalese: 'We need to grab you, we need to grab you', something like that. They definitely tried to abduct me," Mr Perera said.
The assailants fled in a white van when other cars began hooting their horns and a crowd gathered.
No-one has been brought to court for attacks like this on journalists and there are accusations the government has been turning a blind eye, if not encouraging them.
'Sense of responsibility'
The ministry of defence, in particular, objects to what it sees as biased and irresponsible reporting of the war against the Tamil Tiger rebels.
People can't play the fool when it comes to security of a nation. Whatever their write they have to also be responsible
Lt Gen Sarath Fonseka
Sri Lanka's army commander
Articles which have questioned official casualty figures, some military promotions or which have alleged corruption in arms deals have led to some writers being labelled traitors or enemies of the state.
"This concerns the security of the nation," said Sri Lanka's army commander, Lt Gen Sarath Fonseka.
"People can't play the fool when it comes to security of a nation. Whatever they write, they have to also be responsible. They have to write with a sense of responsibility."
Lt Gen Fonseka said newspapers, published by the Tamil Tigers and found in captured bunkers on the frontlines in the north, show how such reporting helps the enemy.
"They have repeated a lot of articles written here in the south by various defence correspondents. That is because they like that kind of article, they know that kind of article is constructive for their war effort, destructive for our war effort. We will publicise that so people will know which articles have become popular with the LTTE," he said.
'Doing their job'
But some have said such actions make journalists potential targets, and have led to self-censorship and a lack of independent reporting.
"The ministry of defence seems to think this demoralises the troops," said Kumar Nadesan, publisher of the Express group of newspapers.
"I don't think they are demoralising the troops. I think the people are solidly behind the president in whatever he is doing, and this is inconsequential, what the journalists are saying.
"But they do need to report on the truth of what is happening. That is their job, that's their function, their duty, their obligation as journalists, nothing more," Mr Nadesan said.
The government has said it is keen to stop attacks on journalists. It has not imposed formal censorship, as has been used intermittently in earlier phases of the long war.
A cabinet sub-committee of ministers has been set up to look into reporters' grievances, and the police have been directed to carry out a full investigation into Monday's attack.
The protesting journalists in Colombo said results would be the proof of the government's sincerity.
Public opinion remains an important battleground in Sri Lanka's civil war.