Gabon's opposition leader Jean Ping has told the BBC a presidential guard helicopter bombed his headquarters and killed two people.
A government spokesman said the operation was to root out "criminals" who had set fire to parliament.
Protestors took to the streets on Wednesday claiming fraud after it was announced that President Ali Bongo had been narrowly re-elected.
Some 1,000 people have been arrested, officials say.
In a national address, Mr Bongo said "democracy does not sit well with an attack on parliament".
Protests and gunfire continued in the capital Libreville on Thursday.
US state department spokesman John Kirby said events in Gabon were being closely monitored by the international community and "appropriate actions" were being considered.
"We deplore the escalation of violence," he said, adding that the US urged "all parties to come together peacefully in this critical time".
The official election result, announced on Wednesday afternoon, gave Mr Bongo a second seven-year term with 49.8% of the vote to Mr Ping's 48.2% - a margin of 5,594 votes.
But Mr Ping said the election was fraudulent and "everybody knows" he won.
Mr Ping won in six out of nine provinces but disputes the result in Mr Bongo's home province of Haut-Ogooue, where turnout was 99.93% and 95% of votes were for the president.
Turnout in the other provinces was between 45% and 71%, according to Gabon's interior ministry.
EU election monitor spokesperson Sarah Crozier told BBC Newsday "it's not a very common result, that's for sure".
Gabon election: Bongo v Ping
- Mr Bongo took office in 2009 after an election marred by violence
- He succeeding his father Omar Bongo who had come to power in 1967 and was Africa's longest serving leader
- Veteran diplomat Mr Ping had served as chair of the African Union
- He had been a close ally of Omar Bongo and had been his foreign minister
- He had two children with Omar Bongo's daughter, Pascaline
Mr Ping has called for voting figures from each polling station to be made public.
The US and EU have also called for the results to be published, while UN Secretary-General Ban Ki-moon has urged calm.
Former colonial power France, which retains strong economic and political ties to the country, also said it was "deeply concerned".
The BBC's Charles Stephane Mavoungou reports from Libreville that on Thursday people in the capital have been unable to access the internet, including social media.
The Boulevard Triomphal, home to Gabon's parliament, was covered in burnt-out cars and lined with torched buildings on Thursday, reports the AFP news agency.
It said police used tear gas to prevent crowds from gathering again and arrested people as they emerged from remains of the parliament.
Police chief Jean-Thierry Oye Zue said six officers had been killed in the protests but declined to give an overall casualty figure, AFP reports.
Oil-rich Gabon has one of the highest per-capita incomes in Africa, but few of its 1.6 million people feel the benefit.