The interior ministry said the deployment that began on Tuesday was aimed at "preserving security and order, and to guarantee an easy flow of traffic".
Al-Wefaq, the main opposition formation, said in a statement that police had blocked roads leading to Shiite-populated villages with barbed wire and concrete blocks.
It accused the authorities of turning some areas into a "vast prison for inhabitants".
Witnesses said the massive deployment has prevented protesters from taking to the streets.
The protest was organised by the Bahrain Rebellion Movement, Tamarod.
It was set up last month, mimicking Egypt's Tamarod movement, which spearheaded the nationwide protests that triggered the army's overthrow of Islamist president Mohamed Morsi.
The Sunni rulers of Shiite-majority Bahrain have assumed sweeping new powers to crush demonstrations, but the opposition activists insist they will go ahead with their planned protest.
On August 1, Tamarod posted an open letter asking the US embassy to provide protection for the rally planned to be held at its doorstep, saying it poses "ethical responsibilities" for Washington.
The group said the goal of the demonstration was a "real democracy in Bahrain not less than that found in Western countries such as the USA and the United Kingdom".
In Washington, Marie Harf, deputy spokeswoman at the State Department, said she was "not going to speculate about what might or might not happen" on Wednesday.
"We support the right of individuals to peacefully assemble and of course, the right of freedom of expression, including in Bahrain, and our support for these principles has not changed, including in Bahrain.
The interior ministry said "terrorists" briefly blocked the Khalifa al-Kabeer highway, in Muharraq governorate, with burned tyres, before security forces cleared the way.
In another statement, it said an Asian fork-lift driver was injured when a "group of terrorists" hurled a petrol bomb at the vehicle while it was clearing a road blocked by protesters.
At least 80 people have been killed in Bahrain since the Arab Spring-inspired protests erupted in early 2011, according to the International Federation for Human Rights.