Azeri President Ilham Aliyev, in power since 2003, is looking to secure another term with Wednesday's election. Last week, European Union leaders expressed concern about the state of political freedom in the Caspian nation.
Human Rights Watch said the official campaign season ended Tuesday but the silence was enforced on opposition voices for much of the year.
"For the past nine months, the government has worked overtime to silence the political opposition," it said in a statement.
The rights groups said at least seven journalists were arrested for what it said were reports deemed critical of the Aliyev administration. British advocacy group Index on Censorship said last week the government had no room for free expression.
The Organization for Security and Cooperation in Europe said Monday there are 390 observers from 42 countries deployed in Azerbaijan. The OSCE said it would issue a statement on the Azeri election Thursday.