The second of three days of discussions were launched by UNESCO director general Irina Bokova who described the library building as "a house of freedom of expression."
"In these times of turbulence freedom of expression has never been more important," Bokova said, noting that over the last 10 years 675 journalists have been killed.
"Only one on 10 cases have been brought to justice," Bokova said.
"In times of change we must remain to true our values... and stand up for them."
Bokova was followed on stage by Latvian Foreign Minister Edgars Rinkevics who described a free press as "the backbone of sustainable democracy and development."
"I would like to express my gratitude to all of you working in this difficult profession... The ability of the media to function independently is vital," Rinkevics said.
He also encouraged the development of "critical thinking and information literacy" as a means of countering the increasingly propagandist output of state-sponsored media in authoritarian regimes.
World Press Freedom Day #WPFD2015 family photo: @edgarsrinkevics @IrinaBokova @PeterGreste #DaceMelbarde #UNESCO pic.twitter.com/iajHbocvXn— Latvian MFA (@Latvian_MFA) May 3, 2015
Later in the day Syrian journalist Mazen Darwish will be awarded UNESCO's main World Press Freedom Award in absentia while he remains incarcerated in Damascus.
The rest of the day's proceedings, which include numerous discussions can be watched online at https://eu2015.lv/.