BEIJING — The police have been dropping in on investment firms and downloading their transaction records. Senior executives at China’s biggest investment bank have been arrested on suspicion of illegal trading. A business journalist has been detained and shown apologizing on national television for writing an article that could have hurt the market.

The Communist Party’s response to China’s monthslong stock market crisis has been swift and forceful. In addition to spending as much as $235 billion to buy shares and bolster prices, the authorities have imposed a range of extraordinary restrictions on the sale of stocks — and backed them with the full weight of a security apparatus usually more focused on political dissent than equity trades.

The strategy appears to have succeeded, at least for now, in softening the impact of the Chinese market’s biggest rout since the global financial crisis of 2008. But the new limits on trading and the efforts by the police and regulators to enforce them have unsettled investors at home and abroad who are unsure exactly what types of transactions are being banned or criminalized.

After decades of watching China make slow but steady progress in building Western-style financial markets, many are now asking whether the party is reversing course — and whether Chinese officials are willing to tolerate the sometimes turbulent waves of selling that are inherent to such markets.