Dozens of boats pulled up around two tankers on lake Maracaibo in western Venezuela, while people on shore gathered in support waving red, yellow and blue Venezuelan flags.
A two-week-old nationwide strike has crippled Venezuela's vital oil industry, and the tankers stalled offshore for days have been a symbol of the opposition's drive to remove Mr Chavez.
The government replaced dissident captains on the striking ships last week, but the tankers still have not moved and the situation on board remains unclear.
The seaborne demonstration came after the opposition held its biggest rally yet on Saturday night, when one million people clogged a main highway in Caracas demanding the President's resignation. Chavez supporters hastily threw a smaller, but boisterous street demonstration of thousands.
The huge rally was a fresh slap to Mr Chavez's elected leftist government as it struggles with the strike launched on December 2 by labour and business leaders bent on toppling Mr Chavez.
The opposition is demanding Mr Chavez resign or call early elections, angered by what it claims is his inept leadership and mishandling of a deteriorating economy.
Mr Chavez, who says he won't resign, was delivering his regular weekend radio and television address, and many were awaiting his response to the rally and to the United States' new stance backing the call for early elections. Venezuela's constitution does not allow for early elections until halfway through a president's term - in this case, August 2003 - and early elections would require amendments to the constitution.
Mr Chavez's leftist ideology, his friendship with Cuba's Fidel Castro and a visit to Iraq have rankled Washington, but Mr Shannon said the Bush administration hopes for a peaceful, democratic electoral solution.
The country has already suffered panic buying and supermarket shortages of basic goods.