"Journalists and media workers have lost 247 of their colleagues over the past six years because of attacks and violations," Emad al-Khafaji, a journalist and writer, told the crowd on Friday.
"The participants in this demonstration have confirmed they will not back down in the face of intimidation and threats."
Website ban considered
Journalists are also concerned about government moves to ban some websites, including those deemed pornographic or to encourage bomb making, prostitution and "terrorism".
Last month, the government passed a bill allowing imported publications to be screened.
"Blocking internet websites and censoring books is a new dictatorship," Muhammad al-Rubaie, a human rights activist at the demonstration, said.
The crowd at Friday's protest, who carried signs saying "do not kill the truth", shouted: "Yes, yes to freedom; no, no to being muzzled".
The Iraqi constitution, which was drawn up in 2005, enshrines freedom of the press and publication unless "public order or morality" is violated.
Ahmed Rushdi, an Iraqi journalist, told Al Jazeera that the government was considering laws to "capture and control any data broadcast, published or uploaded from Iraq".
"This is not acceptable, this is an error in the constitution ... we'll not accept any capturing, we will not accept any silencing, I will not accept to break my pen for [the prime minister] Maliki's sake or any other government," he said.
"Before 2003 there was such silencing, now there will be no silencing."
Under Saddam Hussein, the deposed Iraqi president, the media was heavily censored.
Since the US-led invasion in 2003 that led to Hussein's removal from power, about 200 print outlets, 60 radio stations and 30 television channels have been set up.
But many media outlets are dominated by sectarian and party patrons who use them to push their own agendas.