Shouting "Down with the government", the demonstrators poured into the field and surrounding streets for the rally, organised by the country's major opposition parties.
Thousands of paramilitary troops patrolled the area, police said.
Sheikh Hasina, the opposition leader and former prime minister, is expected to speak later at the rally and announce a new anti-government campaign.
The opposition alliance has boycotted Parliament for more than a year, accusing the government of becoming increasingly autocratic and of corruption, fixing voting lists in its favour and failing to curb rising Muslim extremism.
Police estimated the crowd at about 50,000. Organisers had predicted 100,000 would attend, but thousands of opposition supporters were prevented from entering the capital by barbed-wire barricades erected by police, Abdul Jalil, an opposition spokesman said.
Khaleda Zia, the prime minister,
heads a centre-right coalition
Hasina's opposition Awami League party and its 13 allies also allege that police have arrested more than 2000 activists in Dhaka since Wednesday in an attempt to disrupt the rally.
They have also accused police of blocking buses, trucks and ferries carrying opposition activists towards Dhaka. Police deny the allegations.
MA Quayyum, the inspector-general of police, said suspected criminals were being detained as part of a routine anti-crime campaign, and that no political activists had been taken into custody.
The opposition has held several street protests and general strikes in recent months, demanding the government's resignation and early elections.
But Khaleda Zia's coalition government has vowed to remain in power until its five-year term expires in October.
Bangladesh, an impoverished, officially secular Muslim-majority country of 140 million people, has been rocked by a series of recent bombings that authorities have blamed on an outlawed Muslim group, Jumatul Mujahidin Bangladesh, which wants to establish strict Islamic laws in the country.