"Ukraine will fight this political project, before and after its completion and even after the gas is turned on", presidential spokesman Sergiy Nykyforov said.
The statement was announced after Gazprom confirmed earlier in the day that the construction of the pipeline was completed at 08.45 Moscow time.
Nord Stream 2 is a joint venture of Gazprom and five European partners. The pipeline aims to carry Russian natural gas to Germany across the Baltic Sea. It consists of two 1,230-kilometer (764-mile) long lines with a combined annual capacity of 55 billion cubic meters of gas.
The US and a few countries in Eastern Europe, especially Poland and Ukraine, have actively opposed the construction of Nord Stream 2. The United States, which has attempted to sell its LNG to Europe, has even slapped two rounds of sanctions on the project, claiming that Nord Stream 2 is a threat to European security - which both Moscow and Berlin have denied.