Dealers said the SRB sale had weighed on prices.
"They sold 8,000 lots in the afternoon," a futures dealer in Shanghai said.
The state body was estimated to have sold short on the December and January positions - in each case 4,000 lots, or 20,000 tonnes - through state owned COFCO Futures.
"Of course, the sale will continue," an official for the SRB said when asked whether the state body would continue to sell copper on the market. He added he had received instructions to sell on Monday but declined to give further details.
The official said the SRB, responsible for managing the country's strategic commodity stocks, was releasing copper stocks because the domestic market needed the metal.
Traders estimated the SRB might sell around 100,000 tonnes of copper at home and abroad to rein in record-high prices.
Industry officials last week expected the SRB to sell 80,000 tonnes of copper on the market from its roughly 200,000 tonnes of copper reserves.
"We will sell through many channels. You will see it in few days," the official said. He added the sale channels included the London Metal Exchange and the domestic spot market.
"I have noticed that Shanghai and LME arbitrage is at a record today with Shanghai November being $300 below the LME, accounting for currency and taxes and duties, and December Shanghai is still a whopping $280 below LME," a London-based trader said.
"Two years ago Shanghai was constantly $200-400 above LME," the trader said.
He said the main effect of the SRB sale so far had been to drive down domestic prices. "For the impact on LME, not really visible yet," the trader said.
Traders in China said the SRB was shipping 9,000 tonnes of copper to LME warehouses in Asia.
But the official said the exported shipment was bigger than 9,000 tonnes. "You will see the stocks this week and next week.
"The total volume of the sale may be much bigger than predictions," he said. The same official last week said the SRB could sell as much as 500,000 tonnes of copper.
When asked whether the SRB would empty its copper stocks if it sold 500,000 tonnes, he said: "Only that amount will take all of the SRB's copper stocks? Impossible. Absolutely impossible.
"If they do not believe it, let them. It is other people's judgement," the official said, referring to talk in the market that the SRB might not have 500,000 tonnes of copper in stocks.
The SRB put 20,000 tonnes of copper into Shanghai exchange's warehouses two weeks ago and another 20,000 last week, indicating plans to deliver metal against short positions.
Dealers said the SRB was storing another 15,000 tonnes of copper in an exchange warehouse in Shanghai but had not processed warrants for the copper.
Traders said the SRB had not sold copper on the physical market.
Copper inventories in exchange warehouses jumped 17,490 tonnes, or 37 percent, to 64,840 tonnes in the week ended Thursday. That followed a rise in stocks of 14,485 tonnes, or 44 percent, a week before.