Nino Burjanadze, the country's acting president, said the deal would "raise public trust towards the Georgian Public Broadcaster and the newly introduced standards will be acceptable for all political groups".
Police said about 3,000 people attended the rally while local media said the figure was closer to 7,000.
Many of the crowd wore white scarves, a symbol adopted by Saakashvili's opponents, and shouted anti-government slogans.
Leila Bablidze, an English language teacher at the demonstration, said: "That's our first victory. We'll get many others in the future."
The oppositon has vowed to press on with further street protests and mount a legal challenge to presidential elections which it says Saakashvili rigged in his favour.
Saakashvili, who wants to take the former Soviet republic into Nato, has already met with some opposition figures and promised them places in government.
Official results showed Saakashvili, a 40-year-old US-educated lawyer, won 53 per cent of votes in this month's election, just enough to avoid a second-round run-off.
Tens of thousands of protesters have taken to the streets since the poll, although numbers have fallen short of the opposition's expectations.
Nino Sturua, an opposition spokeswoman, said a complaint would be submitted to a Tbilisi court demanding that the election results be declared void because of vote rigging.
Saakashvili plans to be inaugurated for a second term on January 20, and the government says it opened talks with the opposition on areas such as control over state media to ease tensions.
Saakashvili surged to power in a peaceful 2003 revolution.
He called a snap election following international protests after he crushed anti-government demonstrations violently and declared a state of emergency last year.
International observers said the January 5 election was consistent with most international standards but marred by significant shortcomings such as intimidation and the president's use of his office for campaign purposes.