A network of private and public surveillance cameras, license plate readers and weapons sensors already established in Lower Manhattan as an electronic bulwark against terrorist attacks will soon expand to a large patch of Midtown Manhattan, Mayor Michael R. Bloomberg and Police Commissioner Raymond W. Kelly said Sunday as they announced the allocation of $24 million in Homeland Security grants toward the effort.

Mr. Bloomberg said the expanded monitoring network would cover the areas between 30th and 60th Streets, from the Hudson to the East River.

“We cannot afford to be complacent,” he said, noting that Midtown includes landmarks like Grand Central Terminal, the Empire State Building and the United Nations.

Like the system downtown, the expanded surveillance network would feed streams of data for analysis to a coordination center at 55 Broadway. Mr. Bloomberg, who made the announcement at the center with Mr. Kelly, said work on the Midtown system would begin next year and be completed in 2011.