The demonstration concided with European Commission President Romano Prodi's visit to Riga and the other Baltic capitals, Vilnius and Tallinn.
Demonstrators holding signs saying "No! to EU and global feudalism" said they feared Latvia's loss of sovereignty upon joining the EU and the EU's overriding national legislation, especially that Latvia's language law might be weakened.
"If Latvia joins the EU, it won't be a free country," said Karlis Sils, a student at a Jurmala secondary school. "The EU will be saying each year that the language law must be relaxed more and more and allow more Russian to be used. The law should be stronger, not weaker."
Leaflets passed out by organizers said that the government had been silent on rights given up for EU membership and questioned the benefits.
Leopolds Ozolins held a history book open to the pictures of relatives and others deported from Latvia in 1944 during World War II.
"All Europe and the United States were watching as they emptied villages," Ozolins said.
"It is far from true that all Latvian residents want to become citizens without any rights in a pan-continental organization," Klubs 415 board chairman Valters Grivins said.
KLUBS 415 has about 130 members aged 15 to 30.