Moscow says it will respond if the EU imposes sanctions on Russia over its occupation of Crimea.
The Russian Foreign Ministry said on March 7 that "Russia will not accept the language of sanctions and threats" and that if they are implemented, "they will not be left without a response."
The EU has said it will consider asset freezes and travel bans if the Russian government does not start negotiations with Ukrainian authorities in the "next few days."
The Kremlin also challenged Washington over talk of sanctions.
Russian Foreign Minister Sergei Lavrov warned U.S. Secretary of State John Kerry in a telephone call on March 7 that planned U.S. sanctions against Russia would "inevitably boomerang" on the United States.
Meanwhile, tensions continue to ratchet up on Crimea, where pro-Russia militia are tightening there grip.
Late on March 7, there was a tense standoff between pro-Russian militia and the Ukrainian military at a Ukrainian missile defense post in Sevastopol.
No violence was reported and the militia, reportedly numbering about 20, later left the base.
President Barack Obama discussed the Ukrainian crisis with German Chancellor Angela Merkel in a telephone call late on March 7. The White House said both agreed on the need for Russia to pull back its forces from Crimea.
Earlier in the Russian capital, Moscow police say more than 65,000 people gathered at a rally next to St Basil's Cathedral to support pro-Russian authorities in Crimea who want to join Russia.
The rally, called "We are together," took place just outside the Kremlin walls.
Protester Vera Zaitseva said: "All people living in Crimea consider themselves Russians. For a long time they've been willing to become Russian citizens, because they really didn't like what happened a long time ago, when Crimea stopped being Russian."
The Crimean parliament on March 6 voted in favor of the region becoming part of Russia and scheduled a referendum for March 16.
'No One Will Recognize Referendum'
Meanwhile, Ukrainian Prime Minister Arseniy Yatsenyuk has said that the world will not accept the results of a "so-called referendum" on Crimea joining Russia.
Speaking in Kyiv, Yatsenyuk said: "No one in the civilized world will recognize the results of a so-called referendum carried out by these so-called authorities."
Yatsenyuk said that Ukraine is ready for talks with Russia, but that Moscow must withdraw its troops from Crimea and stop its support for "separatists and terrorists."
Yatsenyuk said he had requested a second phone call with Russian Prime Minister Dmitry Medvedev. Their first phone conversation on March 1 has been the only high-level contact between the two countries since the crisis began.
In Moscow, the heads of both of Russia's parliamentary chambers met with the leader of Crimea's parliament, Vladimir Konstantinov.
Sergei Naryshkin, the speaker of the State Duma, said after the meeting that the lower house will respect the Crimeans' "historic choice."
"We realize that this decision was made as a result of the acute political crisis that is taking place in Ukraine, and it is connected with the effort to ensure the rights and freedoms of citizens, simply to protect human lives," Naryshkin said.
Russian news agency ITAR-TASS quotes Konstantinov as saying that not only Crimea should "be reunified with Russia, but the whole of Ukraine."
Separately, the Organization for Security and Co-operation In Europe (OSCE) says its military observers have again been denied access to Crimea and are heading back to the Ukrainian city of Kherson to plan their next step.
The OSCE's announcement via Twitter came after news agencies reported that armed men at a checkpoint blocked two buses carrying OSCE observers from entering Crimea.
The Russian Foreign Ministry said that the OSCE observers had failed to obtain "official invitations" from the Crimean authorities.
The OSCE team of 47 military and civilian observers was similarly blocked on March 6 and spent the night in Kherson, the nearest big town north of the peninsula.
Based on reporting by AFP and Reuters